Improve `Amount` and `Currency`

[LimpidCrypto]

High Level Overview of Change

Model conversion

This PR changes the lifetime of Currency and Amount from 'static to 'a. It also adds to_amount to turn a Currency into an Amount giving a value. Implement the From<Amount> for Currency trait to turn an Amount into a Currency.

value conversion

The get_value_as_u32 method was also replaced with TryInto<rust_decimal::Decimal> for Amount to have a more convenient way to deal with an Amount.value. The get_value_as_u32 also don't make sense for IssuedCurrencyAmount since it's value is most likely a decimal value.

Type of Change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Refactor (non-breaking change that only restructures code)
• [ ] Tests (You added tests for code that already exists, or your new feature included in this PR)
• [ ] Documentation Updates
• [ ] Release

Future Tasks

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

[LimpidCrypto]

Honestly I think it might be better if we had separate models for XRP and IssuedCurrency as currencies and XRPAmount and IssuedCurrencyAmount as amounts. That enables us to make our library to be stronger typed. Like an AccountRoot objects Balance could only be XRPAmount.

What's your opinion @sephynox ? If you agree I would close the PR and open a new PR soon.

[LimpidCrypto]

Honestly I think it might be better if we had separate models for XRP and IssuedCurrency as currencies and XRPAmount and IssuedCurrencyAmount as amounts. That enables us to make our library to be stronger typed. Like an AccountRoot objects Balance could only be XRPAmount.

What's your opinion @sephynox ? If you agree I would close the PR and open a new PR soon.

We agreed upon separating the currencies/amounts.