Add ser_as_str feature to serialize using serialize_str()

Serializer may not support collect_str, as is the case with
serde_json_core crate.

Cargo.toml

@@ -16,10 +16,12 @@ default = ["std"]
 std = []
 # Implements "schemars::JsonSchema". Also implies "serde".
 json = ["serde", "schemars"]
+ser_as_str = ["heapless"]
 
 [dependencies]
 serde = { package = "serde", version = "1", features = ["derive"], optional = true, default-features=false }
 schemars = { version = "0.8", optional = true }
+heapless = { version = "*", optional = true }
 
 [dev-dependencies]
 serde_test = "1"

src/ipnet_serde.rs

@@ -84,6 +84,13 @@ impl Serialize for Ipv4Net {
         where S: Serializer
     {
         if serializer.is_human_readable() {
+            #[cfg(feature = "ser_as_str")]
+            {
+                let mut buf = heapless::String::<18>::new();
+                fmt::write(&mut buf, format_args!("{self}")).unwrap();
+                serializer.serialize_str(&buf)
+            }
+            #[cfg(not(feature = "ser_as_str"))]
             serializer.collect_str(self)
         } else {
             let mut seq = serializer.serialize_tuple(5)?;
@@ -130,6 +137,13 @@ impl Serialize for Ipv6Net {
         where S: Serializer
     {
         if serializer.is_human_readable() {
+            #[cfg(feature = "ser_as_str")]
+            {
+                let mut buf = heapless::String::<43>::new();
+                fmt::write(&mut buf, format_args!("{self}")).unwrap();
+                serializer.serialize_str(&buf)
+            }
+            #[cfg(not(feature = "ser_as_str"))]
             serializer.collect_str(self)
         } else {
             let mut seq = serializer.serialize_tuple(17)?;