[ccan] [0/3] Generalize and rename memmem module (v2)

[David Gibson]

Recently a series of patches were posted (I forget by whom, sorry),
which added a 'mem' module providing some string.h functions which
aren't available on all platforms.  However, it doesn't seem to have
been reposted.

This patch series implements the same concept in a more minimal way
(for now), renaming the existing 'memmem' module to the more general
'mem' and adding an implementation of memrchr(), which like memmem()
is not standardized though it exists in some C libraries.

Changes in v2:
 * Use 'mem' instead of 'memfuncs' as the module name
 * Fix tests to actually test the function implementations.
 * Fix a bug in memrchr() which meant it wouldn't terminate at the
   front of the given region.