We have some functions which aren't used anywhere. Let's remove them to
get rid of unneeded baggage.

Remove the low-level interface that was exposing implementation details of
`git_idxmap` to callers. From now on, only the high-level functions shall be
used to retrieve or modify values of a map. Adjust remaining existing callers.

Currently, the delete functions of maps do not provide a return value. Like
this, it is impossible to tell whether the entry has really been deleted or not.
Change the implementation to provide either a return value of zero if the entry
has been successfully deleted or `GIT_ENOTFOUND` if the key could not be found.

Convert callers to the `delete_at` functions to instead use this higher-level
interface.

The currently existing function `git_idxmap_resize` and
`git_idxmap_icase_resize` do not return any error codes at all due to their
previous implementation making use of a macro. Due to that, it is impossible to
see whether the resize operation might have failed due to an out-of-memory
situation.

Fix this by providing a proper error code. Adjust callers to make use of it.

Currently, one would use the function `git_idxmap_insert` to insert key/value
pairs into a map. This function has historically been a macro, which is why its
syntax is kind of weird: instead of returning an error code directly, it instead
has to be passed a pointer to where the return value shall be stored. This does
not match libgit2's common idiom of directly returning error codes.

Introduce a new function `git_idxmap_set`, which takes as parameters the map,
key and value and directly returns an error code. Convert all callers of
`git_idxmap_insert` to make use of it.

The current way of looking up an entry from a map is tightly coupled with the
map implementation, as one first has to look up the index of the key and then
retrieve the associated value by using the index. As a caller, you usually do
not care about any indices at all, though, so this is more complicated than
really necessary. Furthermore, it invites for errors to happen if the correct
error checking sequence is not being followed.

Introduce new high-level functions `git_idxmap_get` and `git_idxmap_icase_get`
that take a map and a key and return a pointer to the associated value if such a
key exists. Otherwise, a `NULL` pointer is returned. Adjust all callers that can
trivially be converted.

Currently, the lifecycle functions for maps (allocation, deallocation, resize)
are not named in a uniform way and do not have a uniform function signature.
Rename the functions to fix that, and stick to libgit2's naming scheme of saying
`git_foo_new`. This results in the following new interface for allocation:

- `int git_<t>map_new(git_<t>map **out)` to allocate a new map, returning an
  error code if we ran out of memory

- `void git_<t>map_free(git_<t>map *map)` to free a map

- `void git_<t>map_clear(git<t>map *map)` to remove all entries from a map

This commit also fixes all existing callers.

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

Use the new-style index names throughout our own codebase.