Add predicates for specifying key and value types of maps

[uhoreg]

fixes #57

I used the syntax suggested in #57 (comment) but since I'm using predicates, I wonder if I should add question marks to the names?

[uhoreg]

One problem with using predicates is that it doesn't allow for any changes to be made to the data, as it leaves the data unmodified. e.g. if you do something like values: cast(integer()) |> string(), the values will remain integers, rather than be cast. So this might need to be called out in the docs.

[uhoreg]

superseded by #87