perf: add CowStr type to be able to use Cow::Borrowed on keys

serde by always creates Cow::Owned even if it's possible to Borrow:
serde-rs/serde#1852 (comment)

We create our own wrapper around Cow in order to allow Borrowed keys.

This significantly improves performance:
parse
simple_json
serde_json                               Avg: 283.92 MB/s (-0.84%)     Median: 284.16 MB/s (-0.26%)     [279.00 MB/s .. 289.83 MB/s]
serde_json + access by key               Avg: 276.34 MB/s (-1.24%)     Median: 276.46 MB/s (-1.11%)     [267.63 MB/s .. 284.46 MB/s]
serde_json_borrow                        Avg: 534.96 MB/s (+19.37%)    Median: 533.52 MB/s (+18.98%)    [517.16 MB/s .. 561.15 MB/s]
serde_json_borrow + access by key        Avg: 530.26 MB/s (+18.52%)    Median: 529.16 MB/s (+18.28%)    [519.93 MB/s .. 544.36 MB/s]
SIMD_json_borrow                         Avg: 227.52 MB/s (-0.40%)     Median: 229.19 MB/s (+0.40%)     [184.71 MB/s .. 237.90 MB/s]
hdfs
serde_json                               Avg: 573.25 MB/s (-1.02%)     Median: 573.38 MB/s (-0.78%)     [549.59 MB/s .. 587.74 MB/s]
serde_json + access by key               Avg: 610.33 MB/s (-0.88%)     Median: 609.03 MB/s (-1.00%)     [587.33 MB/s .. 631.20 MB/s]
serde_json_borrow                        Avg: 1.0051 GB/s (+19.36%)    Median: 1.0037 GB/s (+18.06%)    [978.71 MB/s .. 1.0418 GB/s]
serde_json_borrow + access by key        Avg: 1.0453 GB/s (+20.75%)    Median: 1.0432 GB/s (+20.01%)    [1.0123 GB/s .. 1.0802 GB/s]
SIMD_json_borrow                         Avg: 551.42 MB/s (+0.26%)     Median: 554.60 MB/s (+0.91%)     [478.56 MB/s .. 566.50 MB/s]
hdfs_with_array
serde_json                               Avg: 470.84 MB/s (-0.24%)    Median: 470.70 MB/s (-0.20%)    [460.56 MB/s .. 480.44 MB/s]
serde_json + access by key               Avg: 471.89 MB/s (+0.09%)    Median: 471.92 MB/s (-0.03%)    [459.45 MB/s .. 484.84 MB/s]
serde_json_borrow                        Avg: 813.40 MB/s (+7.66%)    Median: 816.02 MB/s (+7.78%)    [798.52 MB/s .. 834.84 MB/s]
serde_json_borrow + access by key        Avg: 821.93 MB/s (+7.64%)    Median: 822.22 MB/s (+7.67%)    [786.16 MB/s .. 838.18 MB/s]
SIMD_json_borrow                         Avg: 458.26 MB/s (-0.15%)    Median: 457.11 MB/s (-0.39%)    [445.08 MB/s .. 473.40 MB/s]
wiki
serde_json                               Avg: 1.2361 GB/s (-0.93%)    Median: 1.2367 GB/s (-1.24%)    [1.2083 GB/s .. 1.2777 GB/s]
serde_json + access by key               Avg: 1.2717 GB/s (-0.63%)    Median: 1.2715 GB/s (-1.91%)    [1.1773 GB/s .. 1.3294 GB/s]
serde_json_borrow                        Avg: 1.4936 GB/s (+3.09%)    Median: 1.4950 GB/s (+3.02%)    [1.4339 GB/s .. 1.5327 GB/s]
serde_json_borrow + access by key        Avg: 1.5255 GB/s (+3.51%)    Median: 1.5266 GB/s (+3.13%)    [1.4733 GB/s .. 1.5849 GB/s]
SIMD_json_borrow                         Avg: 1.2579 GB/s (-1.86%)    Median: 1.2729 GB/s (-1.49%)    [990.58 MB/s .. 1.2995 GB/s]
gh-archive
serde_json                               Avg: 538.63 MB/s (-0.32%)     Median: 539.07 MB/s (-1.29%)     [525.76 MB/s .. 549.05 MB/s]
serde_json + access by key               Avg: 540.55 MB/s (-0.76%)     Median: 541.39 MB/s (-0.84%)     [521.75 MB/s .. 548.68 MB/s]
serde_json_borrow                        Avg: 1.0849 GB/s (+23.13%)    Median: 1.0873 GB/s (+22.41%)    [1.0266 GB/s .. 1.1046 GB/s]
serde_json_borrow + access by key        Avg: 1.0663 GB/s (+20.30%)    Median: 1.0825 GB/s (+22.08%)    [759.26 MB/s .. 1.1250 GB/s]
SIMD_json_borrow                         Avg: 1.0522 GB/s (-0.77%)     Median: 1.0539 GB/s (-0.71%)     [1.0142 GB/s .. 1.0696 GB/s]

src/cowstr.rs

@@ -0,0 +1,37 @@
+use std::{borrow::Cow, ops::Deref};
+
+use serde::Deserialize;
+
+/// A wrapper around `Cow<str>` that implements `Deserialize` and can deserialize
+/// string keys into Cow::Borrowed when possible.
+/// 
+/// This is because serde always deserializes strings into `Cow::Owned`.
+/// https://github.com/serde-rs/serde/issues/1852#issuecomment-559517427
+#[derive(Debug, Clone, PartialEq, Eq, Hash, Deserialize)]
+pub struct CowStr<'a>(#[serde(borrow)] pub Cow<'a, str>);
+
+impl Deref for CowStr<'_> {
+    type Target = str;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl PartialEq<&str> for CowStr<'_> {
+    fn eq(&self, other: &&str) -> bool {
+        self.0 == *other
+    }
+}
+
+impl<'a> From<&'a str> for CowStr<'a> {
+    fn from(s: &'a str) -> Self {
+        Self(Cow::Borrowed(s))
+    }
+}
+
+impl<'a> From<CowStr<'a>> for Cow<'a, str> {
+    fn from(s: CowStr<'a>) -> Self {
+        s.0
+    }
+}

src/de.rs

@@ -145,6 +145,22 @@ mod tests {
 
     use crate::Value;
 
+    #[test]
+    fn cowkeys() {
+        let json_obj = r#"
+            {
+                "bool": true,
+                "escaped\"": true
+            }
+       "#;
+
+        let val: Value = serde_json::from_str(json_obj).unwrap();
+
+        let obj = val.as_object().unwrap();
+        assert!(matches!(obj.as_vec()[0].0.clone().into(), Cow::Borrowed(_)));
+        assert!(matches!(obj.as_vec()[1].0.clone().into(), Cow::Owned(_)));
+    }
+
     #[test]
     fn deserialize_json_test() {
         let json_obj = r#"

src/lib.rs

@@ -78,6 +78,7 @@ mod object_vec;
 mod owned;
 mod ser;
 mod value;
+mod cowstr;
 
 pub use object_vec::{KeyStrType, ObjectAsVec, ObjectAsVec as Map};
 pub use owned::OwnedValue;

src/object_vec.rs

@@ -3,11 +3,11 @@
 
 use std::borrow::Cow;
 
-use crate::Value;
+use crate::{cowstr::CowStr, Value};
 
 #[cfg(feature = "cowkeys")]
 /// The string type used. Can be toggled between &str and Cow<str> via `cowstr` feature flag
-pub type KeyStrType<'a> = Cow<'a, str>;
+pub type KeyStrType<'a> = CowStr<'a>;
 
 #[cfg(not(feature = "cowkeys"))]
 /// The string type used. Can be toggled between &str and Cow<str> via `cowstr` feature flag