Update internal usage of `git_transfer_progress` to
`git_indexer_progreses`.

Currently, our headers need to leak some implementation details of maps due to
their direct use of indices in the implementation of their foreach macros. This
makes it impossible to completely hide the map structures away, and also makes
it impossible to include the khash implementation header in the C files of the
respective map only.

This is now being fixed by providing a high-level iteration interface
`map_iterate`, which takes as inputs the map that shall be iterated over, an
iterator as well as the locations where keys and values shall be put into. For
simplicity's sake, the iterator is a simple `size_t` that shall initialized to
`0` on the first call. All existing foreach macros are then adjusted to make use
of this new function.

There currently exist two different function names for getting the entry count
of maps, where offmaps offset and string maps use `num_entries` and OID maps use
`size`. In most programming languages with built-in map types, this is simply
called `size`, which is also shorter to type. Thus, this commit renames the
other two functions `num_entries` to match the common way and adjusts all
callers.

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

Use the new object_type enumeration names within the codebase.

Instead of using the `khiter_t`, `git_strmap_iter` and `khint_t` types,
simply use `size_t` instead. This decouples code from the khash stuff
and makes it possible to move the khash includes into the implementation
files.

The new connectivity tests are not currently being verified at all due
to being turned off by default. Create two test cases for a pack file
which fails our checks and one which suceeds.

We strive to keep an options structure to many functions to be able to
extend options in the future without breaking the API. `git_indexer_new`
doesn't have one right now, but we want to be able to add an option
for enabling strict packfile verification.

Add a new `git_indexer_options` structure and adjust callers to use
that.