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

In include/git2/odb.h it states that callback can also return
positive value which should break looping.

Implementations of git_odb_foreach() and pack_backend__foreach()
did not respect that.

Use the new object_type enumeration names within the codebase.

In commit 7ec7aa4a7 (odb: assert on logic errors when writing objects,
2018-02-01), the check for whether we are trying to overflowing the fake
stream buffer was changed from returning an error to raising an assert.
The conversion forgot though that the logic around `assert`s are
basically inverted. Previously, if the statement

    stream->written + len > steram->size

evaluated to true, we would return a `-1`. Now we are asserting that
this statement is true, and in case it is not we will raise an error. So
the conversion to the `assert` in fact changed the behaviour to the
complete opposite intention.

Fix the assert by inverting its condition again and add a regression
test.

Our mempack ODB backend has no test coverage at all right now. Add a
simple test suite to at least have some coverage of the most basic
operations on the ODB.

Return an error to the caller when we can't create an object header for
some reason (printf failure) instead of simply asserting.

Only run the large file tests on 64 bit platforms.

Even though we support streaming reads on objects, and do not need to
fit them in memory, we use `size_t` in various places to reflect the
size of an object.

Test that we can read_header on large blobs.  This should succeed on all
platforms since we read only a few bytes into memory to be able to
parse the header.