Implement Hash#except

[wildmaples]

Part of the Ruby 3.0 support - #2453 😊
I followed the specs in - ruby/spec#786

[chrisseaton]

Can we untag anything, or is that not possible until we declare the new version?

[eregon]

Can we untag anything, or is that not possible until we declare the new version?

Yes, using :next and PRETEND_RUBY_VERSION: https://github.com/oracle/truffleruby/blob/master/doc/contributor/workflow.md#running-specs-for-ruby-27-features

Except you need to s/2.7/3.0/ and s/2.6/2.7/ in those docs (I'll try to update them soon, also need to update updating-ruby.md to remind updating that section).

[eregon]

@wildmaples Could you add except_spec.rb to :next and ensures it passes?

[eregon]

Thank you, this looks good.
I'll try to make spec/truffleruby.mspec use the union merge to avoid the conflict.

[eregon]

@wildmaples Could you rebase against latest master?
:next should no longer be changed (you can skip that commit) since RUBY_VERSION is 3.0.2 now.

[eregon]

We don't need this since RUBY_VERSION is 3.0.2 on master now, but no big deal I'll just have a commit on top to revert this small change.
Instead we want to jt untag spec/ruby/core/hash/except_spec.rb, I'll do that on top.