Remove StateBase vtables

[Smattr]

Quoting something from the current sources:

          /* FIXME: This is somewhat awkward. We really want to just do
           * `s[slot] = *t` here. However, we got the memory backing `s` from
           * `calloc` so the vtables of the states were never initialised (to
           * non-null). Of course the assignment operator doesn't write to the
           * vtables because it thinks they've already been initialized. The
           * result is that using the assignment operator here appears to work
           * fine but then later operations that access the vtable segfault. As
           * you can imagine, this was not fun to debug. For now we call the
           * copy constructor to ensure we initialise the vtable, but I think a
           * better long term solution is to make StateBase non-virtual and
           * hence remove its vtable. To do this, we need something else to
           * inherit from BitBlock in place of StateBase and it in turn
           * reference a StateBase.
           */

Having StateBase participate in a class hierarchy is causing problems. Specifically, I didn't anticipate that all virtual calls cannot be collapsed at compile-time (e.g. a parameter to a function that is a reference to an integer that is not known to be in the state vector or in a local variable). Apart from the problem described above, it is highly undesirable for StateBase to have a vtable for allocation reasons. Checking can allocate millions or billions of these and the vtables add up.

We should refactor this area to pare back StateBase to something closer to a C struct with a few helpers.

[Smattr]

Closed in eeb0e2b.