Implement wasm-threads (atomics) proposal

[titzer]

The WebAssembly Threads proposal introduces a set of ~67 bytecodes that perform atomic operations on memory. Though still at Phase 3, this proposal has been implemented in Web browsers and is effectively finished. Wizard should implement these atomic operations, which primarily involves work in the execution tiers. Currently, Wizard already has placeholders for the opcodes in his large Opcode enum--thus work is already scoped.