swift234

Pass #1 at localizing assumptions about fixed layout and
handling non-fixed layouts.

This uncovered a bug where we weren't rounding up the header
size to the element alignment when allocating an array of archetypes.

Writing up a detailed test case for *that* revealed that we were
never initializing the length field of heap arrays.  Fixing that
caused a bunch of tests to crash trying to release stuff.  So...
I've left this in a workaround state right now because I have to
catch a plane.

Swift SVN r4804