Add docs for git_clone_local_t and move the docs for the
git_clone_options to each field.

If the user wants to keep a copy for themselves, they should make a
copy. It adds unnecessary complexity to make sure the returned entries
are valid until the builder is cleared.

Finding a filename in a vector means we need to resort it every time we
want to read from it, which includes every time we want to write to it
as well, as we want to find duplicate keys.

A hash-map fits what we want to do much more accurately, as we do not
care about sorting, but just the particular filename.

We still keep removed entries around, as the interface let you assume
they were going to be around until the treebuilder is cleared or freed,
but in this case that involves an append to a vector in the filter case,
which can now fail.

The only time we care about sorting is when we write out the tree, so
let's make that the only time we do any sorting.

Bump library version to 0.21.0 and SONAME to 21

Since the SOVERSION doesn't need to follow the library's version and
simply needs to be monotonically increasing whenever we release
something that breaks the ABI, we can set some number and allow multiple
versions of the library to be installed side-by-side.

We start here with the minor version as that's what we release for now,
and it allows to backport this change to earlier versions.

This was a bad idea. Don't free except in the free function.