Revise current models

[LimpidCrypto]

There are some improvements need to be done to the current models for transactions and requests:

• utilize Default, PartialEq, Eq, Clone traits
• split up Currency into Currency and Amount
  • Currency::Xrp must have fiel name currency
• add tests for serializing and deserializing models
  • add macro to serialize and deserialize a struct with its tag (e.g. {"StructName": {"foo": 0, "bar": 1}})
• change impl lifetimes
• cleanup models/mod.rs (e.g StreamParameter should be in requests/subscribe.rs)
• add non-common-fields documentation
• look over request_fields.rs and move to models/requests
• Add signing request methods with feature flag
• Add ˋnewˋ methods to all models
  • change tests to use new