This was a public method, so we should probably keep it.

Previously, because opt_aref and opt_aset don't push a frame, when they
would call rb_hash to determine the hash value of the key, the initial
level of recursion would incorrectly use the method id at the top of the
stack instead of "hash".

This commit replaces rb_exec_recursive_outer with
rb_exec_recursive_outer_mid, which takes an explicit method id, so that
we can make the hash calculation behave consistently.

rb_exec_recursive_outer was documented as being internal, so I believe
this should be okay to change.

Asked ko1 about the design.  [ci skip]

Must not be a bad idea to improve documents. [ci skip]

Truly editorial fix for comments.  This works better with Emacs'
set-justification-full function. [ci skip]

Correctly capture thread before releasing GVL and pass as argument to
`rb_thread_scheduler_if_nonblocking`.