contrib/win32/mercurial.ico

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

(binary:image/vnd.microsoft.icon)