Enable conversion of CompoundPeriod to Period

[sostock]

This enables convert(::Type{<:Period}, ::CompoundPeriod).

I did not add support for constructors, because I thought something like Minute(::CompoundPeriod) could be confusing (one could think that it extracts the Minute component from the CompoundPeriod).

This came up in this discourse thread.

[oxinabox]

we should cross-reference this in Dates.toms and Dates.tons.

[sostock]

Sounds good to me, but Dates.toms and Dates.tons (and Dates.days) don’t even have docstrings. Are they considered part of the public API?

[quinnj]

This sounds fine to me; but no, Dates.tons/Dates.toms shouldn't be considered part of the public API. They were meant to be internal and there are probably gotchas if people start using it unawares. We could probably expose something official in the future, but we'd want to put some explicit consideration around it.