0.6.0

A CPython worker can run code set once at capture instead of compiling a source
on every request, and a worker restoring ahead rewrites only the memory the last
request wrote. Workers built on py_reactor.wasm need the new build.

• New wasm_python option entry. Python source the capture runs once;
  it hands worker.set_entry a callable, and a request with no source calls
  it with the context. Nothing is compiled or imported per request: the
  guest's call went from 40 ms to 2 ms for the same request. A request with a
  source still runs it. See docs/python.md.
• The CPython reactor defines its request runner once, in init(), so
  handle() no longer compiles it per request.
• A restore can recycle the last instance's memory. wasm:restore/3
  takes recycle => true: the destroyed instance of the same image in the same
  process gives the next restore its memory, and only the 64 KiB chunks it
  wrote are rewritten. A CPython restore goes from 12 ms to 3.9 ms, and a
  restore_ahead worker, which recycles on its own, answers about 1.7x the
  requests it did. A memory restored this way marks each chunk it writes,
  about 4 ns a store in generated code; everything else pays a field test.
  Generated code is ABI 5, so the compiled tier's disk cache is rebuilt once.
• A restore no longer evaluates the element segments the image
  overwrites.
• The context reaches the CPython reactor through a worker.context
  import. The reactor imports worker.context and worker.context_size,
  so an adapter of your own over py_reactor.wasm has to bind both; images
  of the previous build are not restored (version py-reactor-2).