Reuse parse-partial-sexp state in rainbow-delimiters-propertize.

This is a big performance win, improving both latency and throughput.
Change defsubst -> defun.

There's no real speed gain from using defsubst, and it interferes with
debugging and profiling.
Release version 1.3.9.
Use comment-search-forward to see if a delimiter starts a comment.

(looking-at comment-start-skip) is not always enough, because some
major modes set `comment-start-skip' to not exactly what we expect.

Fixes #35.
Release version 1.3.10.
Test if comment-start-skip is non-nil before using it.
Use when/if instead of and/or+and.
Release version 1.3.11.