Transpose party leaf types

[mitschabaude]

Stacked on #310

Code cleanup PR which mostly affects parties-leaves.ts. Instead of having multiple "modules" like ToFields, ToJson etc, each of which has conversion functions for the core party types like Field, Bool, PublicKey, etc, we "transpose" the code structure, so that we have multiple types, each of which implements methods like toFields, toJson etc. This reduces lots of redundant code since we already implement most of these methods on most of these types, elsewhere.

As a side effect, this PR has to implement the new toInput method (introduced by #310) on CircuitValue, Field, Bool and on the type returned by circuitValue()