Add support for CAST to convert between types

[dantownsend]

There are situations where being able to cast a value to a different type is very useful.

Here we can convert a timestamp to a time:

>>> await Concert.select(
...     Cast(Concert.starts, Time(), "start_time")
... )
[{"start_time": datetime.time(19, 0)}]

In certain (fairly rare) situations, we need to explicitly specify the type of a value in queries, which cast will also help with.