Support timezone object argument to Time.{at,new} and Time#{getlocal,localtime}

[rwstauner]

This adds (at least some) support for the "Timezone Objects" interfaces to several Time methods as described in
https://docs.ruby-lang.org/en/3.3/Time.html#class-Time-label-Timezone+Objects

It was difficult to discern how it all should work but between the CRuby source code and the specs (and tracing what methods were being called on our objects) I think we figured it out.

We started this six months ago as part of HackDays and didn't get very far, then I resurrected it this week.

closes #1717

[andrykonchin]

Thank you! Will review the PR at this a bit later.

[andrykonchin]

Wondering what logic is behind the utc_offset.nonzero? check:

offset = as_local.utc_offset.nonzero? || as_local.to_i - time.to_i

It seems to me it should use either #utc_offset or difference between #to_i values. But it seems not so simple.

[rwstauner]

Wondering what logic is behind the utc_offset.nonzero? check:

offset = as_local.utc_offset.nonzero? || as_local.to_i - time.to_i

It seems to me it should use either #utc_offset or difference between #to_i values. But it seems not so simple.

Originally I seemed to need it... I had to fight a bit with the implementation to make the specs pass because of this trick that happens in the specs:

https://github.com/oracle/truffleruby/blob/9f6f6a7ae38c62bb808c48b0b7560b3fb47f980f/./spec/ruby/core/time/fixtures/classes.rb#L18

Probably once the implementation was right it wasn't needed any more.

The whole thing was a bit confusing. We discovered that in CRuby there's a whole hidden class that gets used for this purpose (Time::tm) which is similar to Time but not quite the same.

Thanks for investigating this and improving it!