First step towards allowing variadic functions

[quephird]

In this PR, we introduce a new ParameterList struct. This will ultimately house both a list of normal parameters as well as a single trailing variadic parameter, but for now will only be used for the former. Doing so takes a first step towards fully supporting variadic functions. The following additional changes were made:

• the lambda case for Expressions and ResolvedExpressions now has an optional ParameterList as an associated value instead of an optional list of Tokens
• in the parser, parseParameters() now constructs and returns a ParameterList. It should be noted that parseFunction() will still initialize a lambda case with a nil value for ParameterList to preserve support for computed properties
• in the resolver, handleLambda() continues to declare and define variables, but this time by iterating over the parameter tokens inside the ParameterList instance it receives
• LoxCallable now has as part of its contract a computed property,parameterList, instead of arity. The three conforming objects, NativeFunction, UserDefinedFunction, and LoxClass, have been updated accordingly, but most notably, NativeFunction synthesizes Token instances with line number values set to 0 in order to return a ParameterList object.
• in the interpreter, functions and lambdas are created with the ParameterList instance passed in instead of [Token], but it is in handleCallExpression() where the arity check is made significantly differently by calling ParameterList.checkArity(argCount:), which will throw if the checkk fails. For the time being ParameterList.checkArity(argCount:) only examines normal parameters.