Separate currency amount/currency

[LimpidCrypto]

This is a follow up #55

High Level Overview of Change

This PR allows us to make our models stronger typed. It adds the following models:

• Currency
  • IssuedCurrency
  • XRP
• Amount
  • IssuedCurrencyAmount
  • XRPAmount

Amounts can be turned into Currencies and vice versa. The value can be easily turned into Decimal using the TryInto trait (error handling still has to be implemented).

Context of Change

https://github.com/sephynox/xrpl-rust/pull/55#issuecomment-1474174141

Type of Change

• [x] Refactor (non-breaking change that only restructures code)

Test Plan

The currencies and amounts are tested in the tests for the models.

Future Tasks

• [ ] Add exceptions as soon as #54 was merged.