Prefer `off64_t` internally.

Our file utils functions all have a "futils" prefix, e.g.
`git_futils_touch`. One would thus naturally guess that their
definitions and implementation would live in files "futils.h" and
"futils.c", respectively, but in fact they live in "fileops.h".

Rename the files to match expectations.

Currently, there is only one caller that adds entries into an offset map, and
this caller first uses `git_offmap_put` to add a key and then set the value at
the returned index by using `git_offmap_set_value_at`. This is just too tighlty
coupled with implementation details of the map as it exposes the index of
inserted entries, which we really do not care about at all.

Introduce a new function `git_offmap_set`, which takes as parameters the map,
key and value and directly returns an error code. Convert the caller to make use
of it instead.

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 a new high-level function `git_offmap_get` that takes a map and a key
and returns 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.

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 a new high-level function `git_oidmap_get` that takes a map and a key
and returns 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.

We use the term "invalid" to refer to bad or malformed data, eg
`GIT_REF_INVALID` and `GIT_EINVALIDSPEC`.  Since we're changing the
names of the `git_object_t`s in this release, update it to be
`GIT_OBJECT_INVALID` instead of `BAD`.

Use the new object_type enumeration names within the codebase.