Add support for a guard in pattern-matching function definition

[evhub]

Should allow

def foo(x is int if x>=0) = x

that is, the general form

def func(pattern if cond):
    body

should be equivalent to

def func(*args):
    match pattern in args if cond:
        body
    else:
        raise MatchError()

[evhub]

Currently live in develop.