midx-write: move writing-related functions from midx.c

midx-write: move writing-related functions from midx.c

Introduce a new midx-write.c source file, which holds all of the
functionality from the MIDX sub-system related to writing new MIDX files.

Similar to the relationship between "pack-bitmap.c" and
"pack-bitmap-write.c", this source file will hold code that is specific
to writing MIDX files as opposed to reading them (the latter will remain
in midx.c).

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>