signature: alloc encoding conversions to Vec/Box

Since signatures all encode to a "bag of bytes" of some form, choosing a
common representation is useful when abstracting over a number of
different signature systems.

These helpers make it easy to use either a `Vec<u8>` or `Box<[u8]>` as
that common type.

signature/src/encoding.rs

@@ -2,6 +2,9 @@
 
 use crate::{Error, Result};
 
+#[cfg(feature = "alloc")]
+use alloc::{boxed::Box, vec::Vec};
+
 /// Support for decoding/encoding signatures as bytes.
 pub trait SignatureEncoding:
     Clone + Sized + for<'a> TryFrom<&'a [u8], Error = Error> + Into<Self::Repr>
@@ -18,4 +21,18 @@ pub trait SignatureEncoding:
     fn to_bytes(&self) -> Self::Repr {
         self.clone().into()
     }
+
+    /// Encode signature as a byte vector.
+    #[cfg(feature = "alloc")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "alloc")))]
+    fn to_vec(&self) -> Vec<u8> {
+        self.to_bytes().as_ref().to_vec()
+    }
+
+    /// Encode the signature as a boxed byte slice.
+    #[cfg(feature = "alloc")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "alloc")))]
+    fn to_boxed_slice(&self) -> Box<[u8]> {
+        self.to_vec().into_boxed_slice()
+    }
 }

signature/src/lib.rs

@@ -130,6 +130,9 @@
 //! [`Digest`]: https://docs.rs/digest/latest/digest/trait.Digest.html
 //! [Fiat-Shamir heuristic]: https://en.wikipedia.org/wiki/Fiat%E2%80%93Shamir_heuristic
 
+#[cfg(feature = "alloc")]
+extern crate alloc;
+
 #[cfg(feature = "std")]
 extern crate std;
 
@@ -145,8 +148,6 @@ compile_error!(
     Use the `rand-preview` feature instead."
 );
 
-#[cfg(feature = "hazmat-preview")]
-#[cfg_attr(docsrs, doc(cfg(feature = "hazmat-preview")))]
 pub mod hazmat;
 
 mod encoding;