Update clar and remove old helpers
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.
Rename the Clay test suite to Clar
Clay is the name of a programming language on the makings, and we want
to avoid confusions. Sorry for the huge diff!