Remove the t operator in type signatures

[billpmurphy]

Instead of having to type t(Maybe, int) or t(Either, "a", str), it would be much cleaner to type Maybe(int) or Either("a", str).

This should be relatively easy to implement, though it would require a major change to some parts of type_system.py, which would be a large breaking change. More experimentation is necessary, I think.

[billpmurphy]

Thinking about this a bit more, it's not possible to remove t entirely, as it's still needed for polymorphic higher-kinded types (e.g. t("m", "a")), but I still think the Maybe(int) syntax is much cleaner than t(Maybe, int).