There has been discussion for a while about making some set of
the `giterr_set` type functions part of the public API for code
that is implementing new backends to libgit2. This makes the
`giterr_set_str()` and `giterr_set_oom()` functions public.
This updates to the latest clar which includes the helpers
`cl_assert_equal_s` and `cl_assert_equal_i`. Convert the code
over to use those and remove the old libgit2-only helpers.
This converts the map validation function into a macro, tweaks
the GITERR_OS system error automatic appending, and adds a
tentative new error access API and some quick unit tests for
both the old and new error APIs.
