Add wrappers to easily deal with the myriad of ScVal integer types.

[Shaptic]

There's some irrelevant yarn fmt changes in here that can be ignored. The main thing to look at is src/numbers/*.js and the corresponding tests. Context: #610. This adds a new API and a helper function:

export type ScIntType =
  | 'i64'
  | 'u64'
  | 'i128'
  | 'u128'
  | 'i256'
  | 'u256';

export class XdrLargeInt {
  constructor(
    type: ScIntType,
    values: number|bigint|string|Array<number|bigint|string>
  );

  toNumber(): number;
  toBigInt(): bigint;

  toI64(): xdr.ScVal;
  toU64(): xdr.ScVal;
  toI128(): xdr.ScVal;
  toU128(): xdr.ScVal;
  toI256(): xdr.ScVal;
  toU256(): xdr.ScVal;

  valueOf(): any; // FIXME
  toString(): string;
  toJSON(): {
    value: string;
    type: ScIntType;
  };
}

export class ScInt extends XdrLargeInt {
  constructor(value: number|bigint|string|ScInt, opts?: { type: ScIntType });
}

export function scValToBigInt(scv: xdr.ScVal): bigint;

Their functionality should be relatively self-explanatory, but you can also refer to the docstrings for details.

[paulbellamy]

Let's merge+release the js-xdr first

[Shaptic]

Mind peeking at stellar/js-xdr#100? 🙏

[sreuland]

this will inherit the same constructor from base class, can skip re-declaring here in the child classes..I think.

[piyalbasu]

Yup, this should inherit the base class constructor automatically.

But doesn't hurt to double check that this still works if you remove it 😅

[Shaptic]

yep I like that it's explicit bc it will document, added jsdocs to make this even more obvious to users

[sreuland]

nice work, left some minor comments for consideration, my approval lighter weight in this area, would look to others from discussion on #610.