- Tested load_bundle, composite types and memory ops.
- Fixed a bug in insertelement.

In theory, all Mu client API functions are available to C, but not all
are tested.  Now a client in C is known to be able to create stack,
thread, handle traps and introspect stacks.

A C client can now call API functions to convert between primitive C
values and Mu handles.

C client can call MuCtx methods and close it now. It looks like it's on
the right track.

Useful for initialising reference fields to pre-allocated objects whose
references are held in global cells.

No C code written.