rust-node: binary Node ID and conversion utilities The choice of type makes sure that a `Node` has the exact wanted size. We'll use a different type for prefixes. Added dependency: hexadecimal conversion relies on the `hex` crate. The fact that sooner or later Mercurial is going to need to change its hash sizes has been taken strongly in consideration: - the hash length is a constant, but that is not directly exposed to callers. Changing the value of that constant is the only thing to do to change the hash length (even in unit tests) - the code could be adapted to support several sizes of hashes, if that turned out to be useful. To that effect, only the size of a given `Node` is exposed in the public API. - callers not involved in initial computation, I/O and FFI are able to operate without a priori assumptions on the hash size. The traits `FromHex` and `ToHex` have not been directly implemented, so that the doc-comments explaining these restrictions would stay really visible in `cargo doc` Differential Revision: https://phab.mercurial-scm.org/D7788

--- a/rust/hg-core/src/revlog.rs	Wed Jan 22 16:23:29 2020 +0100
+++ b/rust/hg-core/src/revlog.rs	Wed Jan 22 16:37:05 2020 +0100
@@ -5,7 +5,9 @@
 // GNU General Public License version 2 or any later version.
 //! Mercurial concepts for handling revision history
 
+pub mod node;
 pub mod nodemap;
+pub use node::{Node, NodeError};
 
 /// Mercurial revision numbers
 ///