Remove sequence from type signatures

[chipotle]

This comes out of discussion in #1170. It seems like it might make sense to explicitly match the input term (e.g., stream, array) with the output type, since a sequence is going to return something specific, not another sequence.

Valid exceptions might be terms that accept any sequence and always return an array, or terms that have no practical way to cover all explicit cases like reduce, which takes a sequence and returns a value.