This introduces the concept of vector index being part of a pointer
type. This avoids vectors having well-defined in-memory layout, and
allows vectors of any integer bit width to work the same way.

When a vector is indexed with a scalar, this is vector element access,
which this branch implements. When a vector is indexed with a vector,
this is gather/scatter, which is not implemented in this branch.

closes #3575
closes #3580