Make Scalar::from_bits a const fn.

const_fn is stable since Rust 1.31
(rust-lang/rust#54835) and enables calling
`Scalar::from_bits(..)` from other const fn contexts, potentially
saving some overhead here and there.

Especially useful in contexts where constants are being built from a bit
pattern.

src/scalar.rs

@@ -243,7 +243,7 @@ impl Scalar {
     /// This function is intended for applications like X25519 which
     /// require specific bit-patterns when performing scalar
     /// multiplication.
-    pub fn from_bits(bytes: [u8; 32]) -> Scalar {
+    pub const fn from_bits(bytes: [u8; 32]) -> Scalar {
         let mut s = Scalar{bytes};
         // Ensure that s < 2^255 by masking the high bit
         s.bytes[31] &= 0b0111_1111;