This is much of the plumbing for the object database to support SHA256,
and for objects to be able to parse SHA256 versions of themselves.

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

RFC: SHA256 proof of concept

Pack files v3 are introduced in the SHA256 hash transition document
https://github.com/git/git/blob/master/Documentation/technical/hash-function-transition.txt

Obviously we do not support these yet. Stop pretending that we do.

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

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

Don't assume that a `git_oid` is a particular size; allocate
`sizeof(git_oid)` instead.

Like we want to separate libgit2 and utility source code, we want to
separate libgit2 and utility tests.  Start by moving all the tests into
libgit2.