Import matrix-multiply from a separate wasm module

[abhi-agg]

For generic matrix multiply interface:

• Restructure marian-dev, allowing it to import different implementations of generic matrix multiply interface for wasm target
• Add a fallback (a polyfill) dummy implementation of this interface in marian-dev for wasm target and export it from marian wasm module
  • The fallback dummy implementation can throw initially
• Change emscripten generated JS glue code to plug in this fallback implementation (until an optimized implementation is not landed in Firefox)
• Verify the restructuring by running wasm artifacts on wasm test page
• Provide a valid (non-dummy) fallback implementations for entire interface

bergamot-translator:

• Make all the necessary changes in bergamot-translator for this feature
• #264

We want to support the fastest configuration --int8shiftAlphaAll