Introduce GrumpkinScalar type

[benesjan]

Introduce a GrumpkinScalar type and use it to represent encryption private key.

Requirements:
Create a GrumpkinScalar type in both Noir and TS. In Noir, the scalar should be represented as 2 fields because the Field type native in Noir is "smaller" than the scalar field of grumpkin. The struct should look like this:

struct GrumpkinScalar {
  low : Field,
  high : Field
}

The get_secret_key oracle call should return it instead of Field.