libgit2 has two distinct requirements that were previously solved by
`git_buf`.  We require:

1. A general purpose string class that provides a number of utility APIs
   for manipulating data (eg, concatenating, truncating, etc).
2. A structure that we can use to return strings to callers that they
   can take ownership of.

By using a single class (`git_buf`) for both of these purposes, we have
confused the API to the point that refactorings are difficult and
reasoning about correctness is also difficult.

Move the utility class `git_buf` to be called `git_str`: this represents
its general purpose, as an internal string buffer class.  The name also
is an homage to Junio Hamano ("gitstr").

The public API remains `git_buf`, and has a much smaller footprint.  It
is generally only used as an "out" param with strict requirements that
follow the documentation.  (Exceptions exist for some legacy APIs to
avoid breaking callers unnecessarily.)

Utility functions exist to convert a user-specified `git_buf` to a
`git_str` so that we can call internal functions, then converting it
back again.

Ensure that the new protection around .git::$INDEX_ALLOCATION rules are
enabled for using the treebuilder when core.protectNTFS is set.

Don't calculate 4 GiB as that will produce a compiler warning on MSVC.
Just hardcode it.

Don't try to feed 4 GB of data to APIs that only take a `size_t` on
32-bit platforms.

The majority of functions are named `from_something` (with an
underscore) instead of `fromsomething`.  Update the index functions for
consistency with the rest of the library.

The `p_fallocate` platform is currently in use in our tests,
only, but it proved to be quite burdensome to get it implemented
in a cross-platform way. The only "real" user is the test
object::tree::read::largefile, where it's used to allocate a
large file in the filesystem only to commit it to the repo and
read its object back again. We can simplify this quite a bit by
just using an in-memory buffer of 4GB. Sure, this cannot be used
on platforms with low resources. But creating 4GB files is not
any better, and we already skip the test if the environment
variable "GITTEST_INVASIVE_FS_SIZE" is not set. So we're arguably
not worse off than before.