Impl TryFrom<&[u8]> for all compressed point types.

This reduces copy-pasta in downstream users to check the length of the
slice beforehand.

src/edwards.rs

@@ -327,12 +327,24 @@ impl Default for CompressedEdwardsY {
     }
 }
 
+impl TryFrom<&[u8]> for CompressedEdwardsY {
+    fn try_from(bytes: &[u8]) -> Result<CompressedEdwardsY, Err> {
+        if bytes.len() != 32 {
+            return Err();
+        }
+
+        Ok(CompressedEdwardsY::from_slice(bytes))
+    }
+}
+
 impl CompressedEdwardsY {
     /// Construct a `CompressedEdwardsY` from a slice of bytes.
     ///
     /// # Panics
     ///
-    /// If the input `bytes` slice does not have a length of 32.
+    /// If the input `bytes` slice does not have a length of 32.  For
+    /// a panic-safe version of this API, see the implementation of
+    /// `TryFrom<&[u8]`.
     pub fn from_slice(bytes: &[u8]) -> CompressedEdwardsY {
         let mut tmp = [0u8; 32];
 

src/ristretto.rs

@@ -217,6 +217,16 @@ impl ConstantTimeEq for CompressedRistretto {
     }
 }
 
+impl TryFrom<&[u8]> for CompressedRistretto {
+    fn try_from(bytes: &[u8]) -> Result<CompressedRistretto, Err> {
+        if bytes.len() != 32 {
+            return Err();
+        }
+
+        Ok(CompressedRistretto::from_slice(bytes))
+    }
+}
+
 impl CompressedRistretto {
     /// Copy the bytes of this `CompressedRistretto`.
     pub fn to_bytes(&self) -> [u8; 32] {
@@ -232,7 +242,9 @@ impl CompressedRistretto {
     ///
     /// # Panics
     ///
-    /// If the input `bytes` slice does not have a length of 32.
+    /// If the input `bytes` slice does not have a length of 32.  For a
+    /// panic-safe version of this API, see the implementation of
+    /// `TryFrom<&[u8]>`.
     pub fn from_slice(bytes: &[u8]) -> CompressedRistretto {
         let mut tmp = [0u8; 32];