It has come to light that the global bindings generated by wasm2c are
not the globals themselves, but rather pointers to the memory locations
where the globals are stored. I've updated sandbox-bindgen.rb to take
this into account.
This executor has the advantage of being able to work correctly when
there are Ruby stack frames underneath C/C++ stack frames in the stack.
Still need to implement handling Ruby fibers.
We can't have them as normal functions because reentrant calls into the
Ruby API don't work if you do that, i.e. calling into the Ruby API and
that calls mkxp-z's bindings and that calls back into the Ruby API.
