This fixes leakage on panic with arrays & slices. I am using a C-style
for-loop instead of a pointer-based loop because that would be ugly-er
to implement.
Use the trait-environment+type as the key. Note that these
are only invoked on types that live for the entire compilation
(no inference artifacts). We no longer need the various special-case
bits and caches that were in place before.
Drop of arrays is now translated in trans::block in an ugly way that I
should clean up in a later PR, and does not handle panics in the middle
of an array drop, but this commit & PR are growing too big.
