Introduce ParameterList struct

[quephird]

Before tackling #34, I should refactor the parser, resolver, and interpreter to use a new ParameterList struct, which not only contains the list of named parameters to a function, but it should also be the object to be consulted when checking the arity of that function. It will ultimately look something like this:

struct ParameterList {
    var normalParameters: [Token]
    var variadicParameter: Token?
}

... but also with an arity checking method. UserDefinedFunction will need to be updated as well.

[quephird]

Addressed in #45