package unit and quantity expressions

[victhorbi]

With the introduction of the classes Hex (encoding floating points numbers, bigint and bytes and strings), HexInt for integers and HexUInt for nsigned integers, there are the super-classes needed to express Thor quantities and units (as wei).

This tickets aims to remove Hex, Hex0x and Quantity functional constant in favour of the new OOP VeChain Data Model and to convert the logic dealing with gas using classes involving integers only.