In https://github.com/libgit2/libgit2/pull/5723/files/fc46dc06f52f854f74371682f911f13856c68edb#r540092847, I was confused by the semantics of `git_index_find`. The documentation says both that it returns the output value in `at_pos` and that it returns it directly as an `int`. This is incorrect; the return value is only returned via `at_pos`.

This change makes the docs reflect reality. The id and size were never
updated in the entry!

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.

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.

We have various macro, enumeration and structure names that were
introduced (very) early in the project and do not match our current
naming conventions.  For instance: `GIT_IDXENTRY...` flags that
correspond to a structure named `git_index_entry`.

Update these to match the current guidance.  The old macros and
enumeration names are reflected as new macros in order to support
backward compatibility (and do so without warnings for consumers).

Provide a public git_index_iterator API that is backed by an index
snapshot.  This allows consumers to provide a stable iteration even
while manipulating the index during iteration.