Decouple external Simd-Handler

[cassinius]

• de-couple compute graph from SIMD implementations
• define an interface for the external handler (functions: matMul etc.)
• write a default pseudo-simd handler implementing this interface (purely in JS or in Rust/Wasm...)
  => Will be done in the course of our Rust-rewrite...
• find a way to take node-tfjs out of package.json since we don't want it to be installed when installing the npm package
  => Easy, install '@tensorflow/tfjs-node' globally & npm link

[cassinius]

Fulfilled to a momentarily satisfying degree - closed.