This step is needed to easily add iterators to git_config_backend
As well use these new git_strmap functions to implement foreach
* git_strmap_iter
* git_strmap_has_data(...)
* git_strmap_begin(...)
* git_strmap_end(...)
* git_strmap_next(...)
This renamed `git_khash_str` to `git_strmap`, `git_hash_oid` to
`git_oidmap`, and deletes `git_hashtable` from the tree, plus
adds unit tests for `git_strmap`.
