TzDateTime enhancements #1473

[hkalexling]

Closes #1473

Added

• datetime::from_date_tz and datetime::from_time_tz: similar to from_date and from_time, but allow additional timezone information and return TzDateTime
• DateTime.with_gmt_offset and TzDateTime.with_gmt_offset: update timestamp to the provided offset while keeping the date and time values unchanged
• DateTime.to_gmt_offset and TzDateTime.to_gmt_offset: update the date and time values to the provided offset while keeping the timestamp unchanged
• TzDateTime.add_* methods: works like the DateTime counterparts but timezone aware

Also included unit tests for the new functions

[lerno]

Can you add a line in the release notes as well?

[hkalexling]

It's already there ;) https://github.com/c3lang/c3c/blob/8ce30bf39dd04ebc56b9ba06a353ac240733a544/releasenotes.md

[lerno]

Oh, sorry, I didn't see it for some reason!

[lerno]

Thank you!