Add alias pattern.
Add finally clause to do blocks.
Add exception throw/catch.
Add new expression.
Convert for loop to use destructuring pattern.
Modify for construct.

The `..' operator is now the keyword `to', and the `...' operator is now
the keyword `until.

Additionally, for loops now take a comma-separated list of generators,
which are consumers of some iteratable. In the future we should allow
guards as well as (arbitrary?) expressions to be mixed in with
generators, allowing something more expressive. Not sure where this
restriction should go (syntactic/semantic analysis).
Remove multiple for generators.

This syntax may be reconsidered once we figure out the features for
generator comprehensions.
Add variadic functions.
Add named arguments.