This is a stable feature release.

Notable changes:

- Introduce hash table improvement (by Vladimir Makarov)
- Binding#irb: Start a REPL session similar to binding.pry
- Unify Fixnum and Bignum into Integer
- String supports Unicode case mappings
- Performance improvements
- Thread#report_on_exception and Thread.report_on_exception changes
- Thread deadlock detection now shows threads with their backtrace and dependency
- Support OpenSSL 1.1.0 (drop support for 0.9.7 or prior)
- ext/tk is now removed from stdlib Feature #8539
- XMLRPC is now removed from stdlib Feature #12160

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>