Allow list-initialization of registers and address spaces.

[jopperm]

@AtomCrafty I'm proposing this to restore the ability to write down constant registers:

const register unsigned<4> BAR[3] = {1,2,3};

Is the approach ok or is there a smarter way in Xtend?

[AtomCrafty]

LGTM! I should have done this in the first place, but there aren't many tests for declarations yet, so it slipped through the cracks.