Fix stack parity for contEnterHelperThunk

It was leaving the stack at a multiple of 8, which then
crashed in DEBUG builds due to misaligned sse moves.
Fix a reentrancy buglet in arr_to_bool.

My hacking around with non-deterministic finalization found
this segv lurking in our current build; if you destroy an array, that
destroys an object, that has a destructor, we didn't have a reentry
record for the call to arr_to_bool.
Print sources for store instructions as sources

I found it a bit confusing that we draw them as dests.
Change AttachString to copy

This modifies AttachString mode for string creation to immediately
copy the string to smart-malloc'd memory and free the input block.

Since we always copy malloc'd strings to smart-malloc'd memory,
and we require strings created with CopyMalloc to run their destructors,
the only kind of sweepable string is the IsShared (SharedVariant) kind;
this slightly simplifies sweeping strings.  (fewer need sweeping, at
least).