Deduplicate activated configs in the .emscripten file (#198)

While doing so, we:

* keep the latest activation (e.g., the user may have activated latest and then latest-upstream, then the upstream LLVM is what is desired).
* keep the order of keys fixed (so the relative order of lines in the .emscripten file is fixed)

This adds some assertions in the Dockerfile, to verify we have one LLVM_ROOT command, and it is the right one.

Fixes #194

Dockerfile

@@ -20,6 +20,8 @@ RUN cd /root/ \
  && /root/emsdk/emsdk activate latest-upstream \
  && source /root/emsdk/emsdk_env.sh --build=Release \
  && emcc hello_world.cpp -s WASM_OBJECT_FILES=1 \
+ && python -c "import os ; assert open(os.path.expanduser('~/.emscripten')).read().count('LLVM_ROOT') == 1" \
+ && python -c "import os ; assert 'upstream' in open(os.path.expanduser('~/.emscripten')).read()" \
  && echo "test fastcomp (waterfall)" \
  && /root/emsdk/emsdk install latest-fastcomp \
  && /root/emsdk/emsdk activate latest-fastcomp \
@@ -28,4 +30,3 @@ RUN cd /root/ \
  && emcc hello_world.cpp -s WASM=0 \
  && echo "test binaryen source build" \
  && /root/emsdk/emsdk install --build=Release binaryen-master-64bit
-