The current map implementations directly include the "khash.h" headers
into their own headers to make available a set of static functions,
defines et cetera. Besides leaking the complete khash namespace into
files wherever khashes are used, this also triggers Clang's
-Wunused-function warnings when some of the static functions are not
being used at all.

Fix the issue by moving the includes into the respective map
implementation files. Add forward declares for all the map types to make
them known.