Update geckodriver to 0.21.0

Fix #93 by avoiding use of TextDecoder

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.

Skip compile and configure cases during numpy build

Obsoletes #22.  Include all codecs, since removing tests imp. size

Update emscripten to 1.38.10

Use ccache to speed up rebuilds

MAINT Minor fixes in the Makefile for make clean

Decrease Python call stack size so it doesn't exceed JS call stack

Fix rebuilding of emsdk