Added keccak_sponge_pad to keccak.h.in
Minor updates and fixes.

build.ninja: Ignore whitespace and blank lines when diffing the KATs.
keccak.h.in: Add prototype for keccak_sponge_absqueeze.
keccak/constructions/sponge.h.in: Add prototype for keccak_sponge_pad.
keccak/rte/check-impl.h: Add checkrsize and checkbuf macros.