content_hash: make public

I'm going to use `content_hash` in a new commit backend for use in
tests.

lib/src/content_hash.rs

@@ -1,3 +1,5 @@
+//! Portable, stable hashing suitable for identifying values
+
 use blake2::Blake2b512;
 use itertools::Itertools as _;
 
@@ -9,9 +11,11 @@ use itertools::Itertools as _;
 /// hash a 32-bit little-endian encoding of the ordinal number of the enum
 /// variant, then the variant's fields in lexical order.
 pub trait ContentHash {
+    /// Update the hasher state with this object's content
     fn hash(&self, state: &mut impl digest::Update);
 }
 
+/// The 512-bit BLAKE2b content hash
 pub fn blake2b_hash(x: &(impl ContentHash + ?Sized)) -> digest::Output<Blake2b512> {
     use digest::Digest;
     let mut hasher = Blake2b512::default();

lib/src/lib.rs

@@ -18,7 +18,7 @@
 #![deny(unused_must_use)]
 
 #[macro_use]
-mod content_hash;
+pub mod content_hash;
 
 pub mod backend;
 pub mod commit;