Make infinite recursion during initialization less likely.
LOOKUP_CHOICE API tweak.
skit_trie is no longer directly referenced.
skit_peg_lookup_index is used instead, and this also allows
(potentially) correct multithreading, which has been implemented
with some posix thread's mutexes.
Add the 'non_whitespace' rule to the PEG-parsing module.
Name change for legibility purposes.
PEG parser improvements.

- Make most PEG operators accept up to 10 arguments.  Before this
    commit, some would only take 5 or 6.
- It is now possible to use SKIT_PEG_DEFINE_RULE to define rules
    with no arguments.
- Better error messages if SKIT_PEG_ASSERT_* receive something
    besides a (skit_peg_parser*) for their first argument.