YJIT: Let RubyVM::YJIT.enable respect --yjit-stats

YJIT: correctly handle case where there are no cfunc calls

Fix bug in top cfunc logging in `--yjit-stats`

* YJIT: gather call counts for individual cfuncs

Co-authored by Takashi Kokubun

* YJIT: record num_send_cfunc stat

Also report num_send_known_cfunc as percentage of num_send_cfunc

* Rename num_send_known_cfunc => num_send_cfunc_inline

Name seems more descriptive of what we do with out custom codegen

* Weaken notice about API stability. A few public APIs in here now.
* Prune out APIs from the docs that are private in nature
* Enable markdown mode and ensure `--` options are quoted so they are
  rendered as two dashes in the HTML.

Replace enumerators with simpler and faster version that only inserts commas before '.' or end of integer string.

* YJIT: Inline basic Ruby methods

* YJIT: Fix "InsnOut operand made it past register allocation"

checktype should not generate a useless instruction.

String#chars returns an array instead of an enumerator since Ruby 2.0.

For <https://bugs.ruby-lang.org/issues/19921>, I suspect the test is
failing due to a timing related interrupt, which on paper could
happen with slow-enough GC runs.

In any case, it's helpful for debugging to have more information when
tests fail.

Co-authored-by: Takashi Kokubun <takashikkbn@gmail.com>