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.

Support reading and writing index v4.  Index v4 uses a very simple
compression scheme for pathnames, but is otherwise similar to index v3.

Signed-off-by: David Turner <dturner@twitter.com>

When we do not trust the on-disk mode, we use the mode of an existing
index entry.  This allows us to preserve executable bits on platforms
that do not honor them on the filesystem.

If there is no stage 0 index entry, also look at conflicts to attempt
to answer this question:  prefer the data from the 'ours' side, then
the 'theirs' side before falling back to the common ancestor.

Find the first index entry matching a prefix.