Try to optimize StableBTreeMap through alternative JSON implementations

[lastmjs]

We might be able to use C++ or Rust JSON implementations like https://github.com/simd-lite/simd-json, which might provide huge speedups in our StableBTreeMap performance, though I have not tested it yet.

There are also some implementations on npm that claim to be very fast versions of JSON.stringify