reftable/basics: merge "publicbasics" into "basics"

reftable/basics: merge "publicbasics" into "basics"

The split between "basics" and "publicbasics" is somewhat arbitrary and
not in line with how we typically structure code in the reftable
library. While we do indeed split up headers into a public and internal
part, we don't do that for the compilation unit itself. Furthermore, the
declarations for "publicbasics.c" are in "reftable-malloc.h", which
isn't in line with our naming schema, either.

Fix these inconsistencies by:

  - Merging "publicbasics.c" into "basics.c".

  - Renaming "reftable-malloc.h" to "reftable-basics.h" as the public
    header.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>