Added `git_repository_submodule_cache_all` to initialze a cache of
submodules on the repository so that operations looking up N
submodules are O(N) and not O(N^2).  Added a
`git_repository_submodule_cache_clear` function to remove the cache.

Also optimized the function that loads all submodules as it was itself
O(N^2) w.r.t the number of submodules, having to loop through the
`.gitmodules` file once per submodule.  I changed it to process the
`.gitmodules` file once, into a map.

Signed-off-by: David Turner <dturner@twosigma.com>