DEV: Split `Currency` into unique structs

[LimpidCrypto]

I find it easier to work with currencies and currency amounts if they were unique structs. So you would have the following structs: Currencies:

• IssuedCurrency
• XRP

Amounts:

• IssuedCurrencyAmount
• XRPAmount

Currently I am having issues accessing the Currency amount but maybe you know a way how to write a get_amount_value function. The way I am suggesting is how other libraries do it (except for the XRPAmount). You can then wrap the currencies in a Currency enum and the amounts in a Amount enum.

[LimpidCrypto]

nvm, found a way :) will be added with #9