Define function composition for date/time values

[eemeli]

This is a follow-up to #798 and the conversations around that.

The intent with this PR is to define what happens when a :datetime, :date, or :time annotated value is used as an operand or option value. It might be best if we didn't initially nitpick on the exact verbiage used in this PR, but rather the overall behaviour: how operand options are merged with expression options, and what values are presented to the consumer.

Given how we've explicitly left out e.g. calendar and timeZone, the language needs to allow for those (and other implementation-defined options) to pass through while being far more opinionated about the style & field options.