Add BigInt composite

[jacogr]

Needs polkadot-js/common#1716

Last piece for issues such as #5706, #3046, #490

TODO:

• We may want to convert the standard Int to also use a wrapper (like here) - it is obviously a major breaking change, however does prepare the road for swap in the future

[spirinvladimir]

It would be a bit more efficient to place validation before creating instance of a class because in case invalid state no extra operations for fill instance properties.

Suggested change

	this.#bitLength = bitLength;
	this.#bitLengthInitial = this.$.toString(2).length;
	this.#encodedLength = this.#bitLength / 8;
	this.isUnsigned = !isSigned;
	const isNegative = this.$ < _0n;
	const maxBits = bitLength - (isSigned && !isNegative ? 1 : 0);
	if (isNegative && !isSigned) {
	throw new Error(`${this.toRawType()}: Negative number passed to unsigned type`);
	} else if (this.#bitLengthInitial > maxBits) {
	throw new Error(`${this.toRawType()}: Input too large. Found input with ${this.#bitLengthInitial} bits, expected ${maxBits}`);
	}
	if (this.$ < _0n && !isSigned) {
	throw new Error(`${this.toRawType()}: Negative number passed to unsigned type`);
	}
	this.#bitLengthInitial = this.$.toString(2).length;
	if (this.#bitLengthInitial >= maxBits) {
	throw new Error(`${this.toRawType()}: Input too large. Found input with ${this.#bitLengthInitial} bits, greater ${maxBits}`);
	}
	this.#bitLength = bitLength;
	this.#encodedLength = this.#bitLength / 8;
	this.isUnsigned = !isSigned;