The experimental function signature is only available when
`GIT_EXPERIMENTAL_SHA256` is enabled.

libgit2 can be built with optional, experimental sha256 support. This
allows consumers to begin testing and providing feedback for our sha256
support while we continue to develop it, and allows us to make API
breaking changes while we iterate on a final sha256 implementation.

The results will be `git2-experimental.dll` and installed as
`git2-experimental.h` to avoid confusion with a production libgit2.

Tidy up `nfmt` / `pathfmt`.

Allow the object database to take an oid type that it supports.  This
oid type will be used to validate the objects that the backends provide.

We intentionally separate oid types from hash types; a hash is a generic
hunk of bytes, an object id has meaning and backs an object on disk.  As
a result of this separation, we need a 1:1 mapping.

`git_oid`s now have a type, and we require the oid type when creating
the object id from creation functions.

Provide helper functions to provide information about the object id size
given its type.

In preparation for SHA256 support, `GIT_OID_RAWSZ` and `GIT_OID_HEXSZ`
need to indicate that they're the size of _SHA1_ OIDs.