Snarkyjs: Reduce loading time

[mitschabaude]

supports o1-labs/o1js#1073

• make computation of field size bits lazy so it doesn't block web page loading
  • Z.testbit is very slow. In the near future I hope to put more performant js implementations in place, and changes like this one will no longer be necessary
• use Field.to_string and Field.of_string from the bindings instead of implementing it via to_bignum_bigint and Bignum_bigint.to_string
  • this just makes sense, because it gives us the flexibility of optimizing the implementation in JS
  • just a glimpse of many future similar changes that I'd like to do, avoiding the zarith bigint stubs which are slow