perf: don't use mutex

[dmitryvk]

For trivial streams, Mutex shows up in flamegraphs. We can avoid using the mutex, since streams are inherently single-threaded and it's only valid to access the emitter from within the stream's closure.

• Make the runtime check of validity access stricter
• Run tests under Miri since we're using unsafe now
• Use thread-local pointer to validate shared state accesses instead of a mutex