Improve documentation

The function `git_treebuilder_write_with_buffer` is unnecessary; it
is used internally as part of treebuilder writing, but it has little
use to external callers.  For callers that repeatedly write a
treebuilder, we can supply them with a buffer in the treebuilder struct
instead of recreating it.  For ourselves, when we want a single buffer
in our write loop, we can use an internal function.

Stop returning a void for functions, future-proofing them to allow them
to fail.

We keep the treebuilder entrycount as a `size_t` - return that instead
of downcasting to an `unsigned int`.  Callers who were storing this
value in an `unsigned int` will continue to downcast themselves, so
there should be no behavior change for callers.

Since we now always build the library with cdecl calling conventions,
our callbacks should be decorated as such so that users will not be able
to provide callbacks defined with other calling conventions.

The `GIT_CALLBACK` macro will inject the `__cdecl` attribute as
appropriate.

git_object_t is the future; update the public API to use it.  This will
also ensure that we can build our tests which make use of the old API
without modification (and without compiler warnings).