Put correct VWord width tag on result of symbolic index operation.
Put correct VWord width tag on result of symbolic index operation.

Fixes #474.

Previously, applying (@) at type `[a][b] -> [i] -> [b]` would tag the
result value as having width `a` instead of width `b`. This is now fixed.

GaloisInc/cryptol,GaloisInc/cryptol,GaloisInc/cryptol