refactor: Rewrite to CurrencyCache

[ZeroWave022]

This pull request removes CacheHandler and replaces it with CurrencyCache.

Side effects:

• The client now has cryptos and fiats properties, giving access to a CurrencyCache each.
• Client.getCryptoCurrencies and Client.getFiatCurrencies are removed, as you can now access these values without calling any function.
• Amount does not use the cache anymore, as it is now managed by the client.

CryptoCurrency and FiatCurrency receive helper functions:

• convertToAmount goes from a raw API value to a readable one.
• convertToRaw goes from a readable value to a raw API one.