Optional and variadic function arguments

[mbeckem]

Currently user defined functions can only specify a fixed number of parameters:

fn foo(a, b, c) { ... }

We should at least support optional function parameters and possibly also variadic functions.

Example syntax for optional parameters:

fn dump(value, pretty = false) { ... }

"?" syntax like in TypeScript (e.g. pretty?) is not a good idea here, i think.
The only fitting default value would be null (i do not want an undefined construct exposed to the language).