Add concurrent mruby VMs and bytecode

[jackalcooper]

What changed

• run independent VMs concurrently on dirty CPU schedulers while serializing each individual VM
• add compiled RITE bytecode resources reusable across isolated VMs
• make bytecode close safe during concurrent use
• validate VM isolation, concurrency, malformed compilation, and bytecode reuse

Why

This turns the package from one-shot evaluation into a practical multi-instance embedded runtime with reusable compiled programs.

Validation

• MIX_ENV=test mix compile --force --warnings-as-errors
• mix credo --strict
• mix test --force (9 tests)

Stacked on #94.