Add some missing trap cases

[lukewagner]

Alex spotted missing memory length trap_ifs in string lowering, so I did a little audit and spotted a few other discrepancies:

• string lifting doesn't check the same alignment that it requires for lowering
• load()/store() already implicitly assume in some cases (records/variants) that someone "higher up" checked the whole contiguous allocation, so assert that and remove the superfluous trap_ifs in the integer cases. Auditing callers, the "too many params/results" case was missing this check