Improve error when reading variable in provable code

[mitschabaude]

closes #899
closes #478
closes #489

bindings: o1-labs/o1js-bindings#52

Consider this method which incorrectly tries to extract the JS value of a variable:

  @method update(y: Field): Field {
    console.log(y.toString());
    // ...
 }

We used to get an error that confused people again & again:

Error: Can't evaluate prover code outside an as_prover block

This frequently caused people to wrap their entire logic in Provable.asProver(...) 😬

With this PR, the error now looks like:

Equivalent error messages happen in methods such as Bool.toBoolean(), Field.toBigInt() and PublicKey.toBase58(). We took care to adapt the error message to refer to a method that a user likely called themselves - e.g., refer to PublicKey.toBase58() in the error message even though it causes the error by internally calling Field.toBigInt().