Add common hash sizes From impls

[parazyd]

blake2s:
* From<[u8; 16]>

blake2b:
* From<[u8; 16]>
* From<[u8; 32]>

Both introduce an intermediate buffer in the implementation, but this is a small tradeoff that allows usage of some common hash sizes more idiomatically for the users of this library.

[parazyd]

@oconnor663 Any plans to check this out?

[oconnor663]

Thanks, this looks like a good idea. What would you think about doing it generically, something like this:

diff --git a/blake2b/src/lib.rs b/blake2b/src/lib.rs
index 6c4f92f..aa58385 100644
--- a/blake2b/src/lib.rs
+++ b/blake2b/src/lib.rs
@@ -616,17 +616,20 @@ fn bytes_to_hex(bytes: &[u8]) -> HexString {
     s
 }
 
-impl From<[u8; OUTBYTES]> for Hash {
-    fn from(bytes: [u8; OUTBYTES]) -> Self {
+impl<const N: usize> From<[u8; N]> for Hash {
+    fn from(bytes: [u8; N]) -> Self {
+        assert!(N <= OUTBYTES, "Hash is at most {OUTBYTES} bytes");
+        let mut buf = [0u8; OUTBYTES];
+        buf[0..N].copy_from_slice(&bytes);
         Self {
-            bytes,
-            len: OUTBYTES as u8,
+            bytes: buf,
+            len: N as u8,
         }
     }
 }
 
-impl From<&[u8; OUTBYTES]> for Hash {
-    fn from(bytes: &[u8; OUTBYTES]) -> Self {
+impl<const N: usize> From<&[u8; N]> for Hash {
+    fn from(bytes: &[u8; N]) -> Self {
         Self::from(*bytes)
     }
 }
diff --git a/blake2s/src/lib.rs b/blake2s/src/lib.rs
index f845e96..c2852b4 100644
--- a/blake2s/src/lib.rs
+++ b/blake2s/src/lib.rs
@@ -608,17 +608,20 @@ fn bytes_to_hex(bytes: &[u8]) -> HexString {
     s
 }
 
-impl From<[u8; OUTBYTES]> for Hash {
-    fn from(bytes: [u8; OUTBYTES]) -> Self {
+impl<const N: usize> From<[u8; N]> for Hash {
+    fn from(bytes: [u8; N]) -> Self {
+        assert!(N <= OUTBYTES, "Hash is at most {OUTBYTES} bytes");
+        let mut buf = [0u8; OUTBYTES];
+        buf[0..N].copy_from_slice(&bytes);
         Self {
-            bytes,
-            len: OUTBYTES as u8,
+            bytes: buf,
+            len: N as u8,
         }
     }
 }
 
-impl From<&[u8; OUTBYTES]> for Hash {
-    fn from(bytes: &[u8; OUTBYTES]) -> Self {
+impl<const N: usize> From<&[u8; N]> for Hash {
+    fn from(bytes: &[u8; N]) -> Self {
         Self::from(*bytes)
     }
 }