Any use of TextDecoder on a part of the Wasm HEAP seems to cause crashes
when using WebAssembly.instantiate later on (in Chrome).

The fix is to:

- Avoid use of TextDecoder in emscripten-generated code by setting
  -s TEXTDECODER=0
- Do the Python-to-JS string conversion a different way:

  - Handle the native format of a Python Unicode string (which is either UCS1,
    UCS 2 or UCS 4) directly.  This has the added advantage of being
    computationally simpler than encoding/decoding to/from UTF8.