Keyword parameters for MutableStruct in Ruby 2.5.0

[ilyacherevkov]

Ruby 2.5.0 introduced Struct keyword parameters by adding "keyword_init: true" optional parameter.
Mutable / ImmutableStruct / SettableStruct don't have support for those.

[pitr-ch]

Thanks for the suggestion.

[numeraltwo]

Hey @pitr-ch I've got something working for this if you are still looking for a contributor. If so, I'll refine and make a PR in the next few days.

[pitr-ch]

@numeraltwo that would be great 👍 Please go ahead and create a PR.