Improving built-in function sequence types

[CanBerker]

Sequence types for built-in functions do not completely cover cartesian product of type-arity combinations. Furthermore, if a non-existing sequenceType is referenced, this causes nullpointer exceptions without a good error handling.

An idea by @ghislainfourny: Maybe use enums rather than strings would make the code more robust to avoid any null pointer exceptions

Originally discussed in #481 (comment)