♻️ Change Currency to class

[greymistcube]

• Currency structure is too complicated to be used as a struct.
• Moreover, the size of Currency as a struct stands at 72 bytes, which is too large.
• The default for Currency is an invalid value where it can break things down the line.

Also I've cleaned up most constructors to have consistent validation logic for creating a new Currency by funneling them to a single private constructor. However, there still is a loophole where an invalid currency can be created through JSON deserialization. This is due to the JSON format of Currency having a yet another scheme that is different from the one for IValue or for Hash. 🙄