Add structProvable

[mrmr1993]

This PR introduces structProvable, an easy way to build a GenericProvable for an object with fields only.

Compiling example:

type MyType<X, Y, Z> = {
    x: X,
    y: Y,
    z: Z,
}

type MyVarType<X, Y, Z> = {
    x: X,
    y: Y,
    z: Z,
}

function myTyp<X, XValue, Y, YValue, Z, ZValue, Field>(
    xtyp: GenericProvable<X, XValue, Field>,
    ytyp: GenericProvable<Y, YValue, Field>,
    ztyp: GenericProvable<Z, ZValue, Field>) {
    structProvable<MyVarType<X, Y, Z>, MyType<XValue, YValue, ZValue>, Field>({x: xtyp, y: ytyp, z: ztyp});
};

type MyRecord = {
    x: [],
    y: [],
    z: [],
}

type MyVarRecord = {
    x: [],
    y: [],
    z: [],
}

function EmptyListType(): GenericProvable<[], [], []> { throw new Error("TODO"); }

let myRecordTyp : GenericProvable<MyRecord, MyVarRecord, []> = structProvable({x: EmptyListType(), y: EmptyListType(), z: EmptyListType()});

Idealised example assuming supporting GenericProvables for the fields:

type MyVarStructure = {
    b: boolean,
    i: Int32,
}
type MyValueStructure = {
    b: Boolean,
    i: Int32,
}

let myStructureTyp : GenericProvable<MyVarStructure, MyValueStructure, Field> =
  structProvable({b: booleanTyp, i: int32Typ});

[mitschabaude]

this already exists as provable() (on which Struct is based) in ./provable-generic.ts

provable() is much more general though, handles arrays and singleton types as well, as well as nested structs / arrays

[mitschabaude]

it's supposed to be called without argument, and return a default value, when we're in compilation mode