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.

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

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.

Add optional refreshing in the fake backend, and count the number of
refresh calls if enabled.

Move to the `git_error` name in the internal API for error-related
functions.

Use the new object_type enumeration names within the codebase.

The fake backend currently implements all reading functions except for
the `exists_prefix` one. Implement it to enable further testing of the
ODB layer.

The `search_object` function takes the OID length as one of its
parameters, where its maximum length is `GIT_OID_HEXSZ`. The `exists`
function of the fake backend used `GIT_OID_RAWSZ` though, leading to
only the first half of the OID being used when finding the correct
object.

In order to be able to test the ODB prefix functions, we need to be able
to detect ambiguous prefixes in case multiple objects with the same
prefix exist in the fake ODB. Extend `search_object` to detect ambiguous
queries and have callers return its error code instead of always
returning `GIT_ENOTFOUND`.

Right now, the fake backend is quite restrained in the way how it
works: we pass it an OID which it is to return later as well as an error
code we want it to return. While this is sufficient for existing tests,
we can make the fake backend a little bit more generic in order to allow
us testing for additional scenarios.

To do so, we change the backend to not accept an error code and OID
which it is to return for queries, but instead a simple array of OIDs
with their respective blob contents. On each query, the fake backend
simply iterates through this array and returns the first matching
object.

The fake backend used by the test suite `odb::backend::nonrefreshing` is
useful to have some low-level tests for the ODB layer. As such, we move
the implementation into its own `backend_helpers` module.