opt_aref_with is an optimized instruction for accessing a Hash using a
non-frozen string key (ie. from a file without frozen_string_literal).
It attempts to avoid allocating the string, and instead silently using a
frozen string (hash string keys are always fstrings).

Because this is just an optimization, it should be invisible to the
user. However, previously this optimization was could be seen via hashes
with default procs.

For example, previously:

    h = Hash.new { |h, k| k.frozen? }
    str = "foo"
    h[str]   # false
    h["foo"] # true when optimizations enabled

This commit checks that the Hash doesn't have a default proc when using
opt_aref_with.

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.

tab->entries_bound is used to check if the bins are full in
rebuild_table_if_necessary.

Hash#shift against an empty hash assigned 0 to tab->entries_bound, but
didn't clear the bins. Thus, the table is not rebuilt even when the bins
are full. Attempting to add a new element into full-bin hash gets stuck.

This change stops clearing tab->entries_bound in Hash#shift.
[Bug #18578]

Fixes [Bug #16908]

[Feature #14579]

The value of the dynamic key cannot be omitted for now.

`{x:, y:}` is a syntax sugar of `{x: x, y: y}`.

This makes the compare_by_identity setting always copied
for the following methods:

* except
* merge
* reject
* select
* slice
* transform_values

Some of these methods did not copy the setting, or only
copied the setting if the receiver was not empty.

Fixes [Bug #17757]

Co-authored-by: Kenichi Kamiya <kachick1@gmail.com>