Feature: Add support for match conditions

[alimpfard]

It'd be neat if we could somehow generate (and optimise for) runtime conditional matches, possibly with base syntax like:

# As a runtime cond
define global x = 0
Foobar :: Foo\C{x < 4}bar

# Or as a 'start condition'
define global state = 0
Foobar :: \C{state ? 0}Foobar\E{state = 1}