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.

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

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.

Use the new object_type enumeration names within the codebase.

The commit message encoding is currently being parsed by the
`git__prefixcmp` function. As this function does not accept a buffer
length, it will happily skip over a buffer's end if it is not `NUL`
terminated.

Fix the issue by using `git__prefixncmp` instead. Add a test that
verifies that we are unable to parse the encoding field if it's cut off
by the supplied buffer length.

We currently do not have any test suites dedicated to parsing commits
from their raw representations. Add one based on `git_object__from_raw`
to be able to test special cases more easily.