erlang_wasm 0.4.0

0.4.0

The workers ship with the application. Running JavaScript, Python or Lua
now needs the dependency and a runtime artifact, and nothing copied from
examples/. Nothing breaks: the new modules take names no 0.3 example used,
so code you copied keeps working. Upgrading from 0.3
maps each copied module to its installed one.

• wasm_script_worker is the worker kernel; run/3 takes a source and a
  context. wasm_worker_adapter is the behaviour an adapter implements.
• The adapters are wasm_javascript, wasm_javascript_command,
  wasm_python, wasm_python_command and wasm_lua.
• wasm_instance_worker is the 0.3 examples/wasm_worker.erl, with the same
  calls.
• The application starts the reaper. Set scratch_roots so a restarted node
  cleans up what a crashed one left, and reaper_options for the cleanup
  limits. wasm_script_worker:cleanup_stats/0 and cleanup_requests/0 show
  what it holds.
• The guides open with what you want to do, and ten example pages run as
  written. Every code block in README.md, docs/ and the module docs is
  checked on every test run, and getting-started.md runs as pasted.