A fairly simple one, only valid on the 'set' construct, this clause
takes an int expression. Most of the work was already done as a part of
parsing, so this patch ends up being a lot of infrastructure.
The 'set' construct is another fairly simple one, it doesn't have an
associated statement and only a handful of allowed clauses. This patch
implements it and all the rules for it, allowing 3 of its for clauses.
The only exception is default_async, which will be implemented in a
future patch, because it isn't just being enabled, it needs a complete
new implementation.
