Avoid polluting BigNumber prototype

[roccomuso]

Since it's used only in 1 place, this can be avoided:

theta-wallet-web/src/libs/thetajs.esm.js

Lines 77 to 81 in e6cf02e

	BigNumber.prototype.pad = function(size) {
	var s = String(this);
	while (s.length < (size || 2)) {s = "0" + s;}
	return s;
	};

and replaced using lodash padStart(x, 2, '0').
Not a good practice to pollute prototype for libs being reused elsewhere in a project.