In #2818, `Hash.ruby2_keywords!` has renamed to `Hash.ruby2_keywords_hash`.

And revert "Stop test-bundled-gems on macOS for now",
28b290f7.

As the only h2 did not have its exclusively owned body text,
merged it to the first paragraph.  Then pulled up h3 and deeper
headings.

* The definition lists extensions of the RDoc Markdown parser does
  not support nesting.

* The RDoc Markdown parser requires more indents for nested lists.

* Remove builtin_binary.inc which is generated for each time
  miniruby is built.

* dSYM is a directory, not a file.

To enable implementation-defined output processing, for the
compatibility with readline.  [Bug #16509]

https://bugs.ruby-lang.org/issues/16509

https://github.com/ruby/io-console/commit/8c8b0b6757

This breaks tests using this path on JRuby because the `jruby`
executable turns into `jjruby` after the sub.

https://github.com/ruby/io-console/commit/e5951aa34c

https://github.com/ruby/io-console/commit/73e7b6318a

There is no active maintainer for cygwin.  The CI failure is too noisy.

[Misc #16407]

It was found that a feature to check and add ruby2_keywords flag to an
existing Hash is needed when arguments are serialized and deserialized.
It is possible to do the same without explicit APIs, but it would be
good to provide them as a core feature.

https://github.com/rails/rails/pull/38105#discussion_r361863767

Hash.ruby2_keywords_hash?(hash) checks if hash is flagged or not.
Hash.ruby2_keywords_hash(hash) returns a duplicated hash that has a
ruby2_keywords flag,

[Bug #16486]

It is useful for a program that dumps and load arguments (like drb).
In future, they should deal with both positional arguments and keyword
ones explicitly, but until ruby2_keywords is deprecated, it is good to
support the flag in marshal.

The implementation is similar to String's encoding; it is dumped as a
hidden instance variable.

[Feature #16501]

internal/rational.h needs internal/warnings.h with Apple clang,
for `UNALIGNED_MEMBER_ACCESS`.

patch from https://travis-ci.org/ruby/ruby/jobs/638231960

patch from https://travis-ci.org/ruby/ruby/jobs/638226493

[DOC] [ci skip] [Bug #8841]

[Feature #16432]

  [Feature #16484][ruby-core:96682]

Split with the matched part when the separator matches the empty
part at the beginning.  [Bug #11014]

`String#sub` with a string pattern defers creating a `Regexp`
until `MatchData#regexp` creates a `Regexp` from the matched
string.  `Regexp#last_match(group_name)` accessed its content
without creating the `Regexp` though.  [Bug #16508]