0.5.0

Requests on a pool of script workers no longer wait on one another in
node-wide processes. Nothing in your code changes. One new worker option.

• New worker option restore_ahead. With a captured image, the worker
  restores the next request's instance while it waits, so a request starts at
  the guest's own work: a CPython request's deliver and restore goes from
  15 ms to 38 us when an instance is waiting. Every request still gets a
  fresh instance. It holds one instance's memory per idle worker, needs
  imports that are all functions, and helps only when workers have idle time
  between requests; see docs/tuning.md.
• File operations on the request path are raw. Staging, mounts, request
  directories, cleanup and WASI path resolution no longer go through
  file_server_2, which a request called about 34 times.
• The reaper does no file I/O of its own. Journal records are written by
  writer processes and are no longer synced; a start removes every request
  directory no record names, which covers a record lost to a host crash.
  DOWN handling is O(1) and the operator view (cleanup_stats/0,
  cleanup_requests/0) is refreshed at most every 50 ms.
• Fewer keeper and code-slot calls per instance: four keeper calls where
  there were six, and two code-slot calls where there were seven for a module
  compiled as one unit.
• wasm_jit:counts/0's entered and reentered are counted per
  scheduler, without a shared word on the call path.