oid: `GIT_OID_*SZ` is now `GIT_OID_SHA1_*SIZE`
In preparation for SHA256 support, `GIT_OID_RAWSZ` and `GIT_OID_HEXSZ`
need to indicate that they're the size of _SHA1_ OIDs.
refactor: `tests` is now `tests/libgit2`
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.