Static or runtime (or both) bounds checking

[pearu]

Static or runtime (or both) bounds checking for reads/writes to make functions safe.

Static bounds checking could be potentially written as an LLVM pass that does simple min/max access range calculations, similar to what we do in ExpressionRange. If the expressions are too complicated to compute, we could fall back to lightweight runtime bounds checking (there is an llvm pass for this, but it looks quite heavy)