Better optional arguments

[irh]

This PR makes it easier to define optional arguments for functions.

Instead of checking for missing arguments in the function body (by checking against null), you can define default values in the argument list.

e.g. Instead of:

f = |a, b|
  a = a or 100
  b = b or 23
  a + b

you can write:

f = |a = 100, b = 23|
  a + b

Now that optional arguments are an explicit language feature, it makes sense for the runtime to be more restrictive and throw an error if a function is called with an unexpected number of arguments.