Move heapam_handler.c index scan code to new file.

Move heapam_handler.c index scan code to new file.

Move the heapam index fetch callbacks (index_fetch_begin,
index_fetch_reset, index_fetch_end, and index_fetch_tuple) into a new
dedicated file.  Also move heap_hot_search_buffer over.  This is a
purely mechanical move with no functional impact.

Upcoming work to add a slot-based table AM interface for index scans
will substantially expand this code.  Keeping it in heapam_handler.c
would clutter a file whose primary role is to wire up the TableAmRoutine
callbacks.  Bitmap heap scans and sequential scans would benefit from
similar separation in the future.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/bmbrkiyjxoal6o5xadzv5bveoynrt3x37wqch7w3jnwumkq2yo@b4zmtnrfs4mh