signature: remove AsMut bound on Repr (#1144)

Without a `Default` bound (#1143) there's little purpose to having an
`AsMut` bound on `Repr`, which would allow `Default` to construct an
empty bytestring and `AsMut` used to write into it.

Given there isn't an obvious use case, fewer bounds are better.

signature/src/encoding.rs

@@ -10,7 +10,7 @@ pub trait SignatureEncoding:
     Clone + Sized + for<'a> TryFrom<&'a [u8], Error = Error> + Into<Self::Repr>
 {
     /// Byte representation of a signature.
-    type Repr: 'static + AsRef<[u8]> + AsMut<[u8]> + Clone + Send + Sync;
+    type Repr: 'static + AsRef<[u8]> + Clone + Send + Sync;
 
     /// Decode signature from its byte representation.
     fn from_bytes(bytes: &Self::Repr) -> Result<Self> {