Add Period type

[Akirathan]

Pull Request Description

This PR adds Period type, which is a date-only complement to Duration builtin type.

Important Notes

• Period replaces Date_Period, and Time_Period.
• Added shorthand constructors for Duration and Period. For example: Period.days 10 instead of Period.new days=10.
• Period can be compared to other Period in some cases, other cases throw an error.

Checklist

Please include the following checklist in your PR:

• The documentation has been updated if necessary.
• All code conforms to the
  Scala,
  Java,
  and
  Rust
  style guides.
• All code has been tested:
  • Unit tests have been written where possible.
  • If GUI codebase was changed: Enso GUI was tested when built using BOTH
    ./run ide build and ./run ide watch.

[hubertp]

Minor nits, looks OK otherwise

[JaroslavTulach]

Fine from my "engine" perspective.

[4e6]

Looks good!

[jdunkerley]

Generally looks good but am not sure about the == and compare_to - adding the starting date feels wrong to me.

[jdunkerley]

LGTM - will want to add these types to default Base exports but will do as part of the great tidy up...