Experimental JsonReader::next_number_int

[Marcono1234]

Based on #158 Struson 0.8 will include a new experimental method JsonReader::next_number_int. This method is intended for parsing integer JSON numbers and can be potentially more performant than next_number or next_number_as_str.

This discussion here is for collecting your feedback on this new method, for example:

• Do you like or don't like the method, why?
• Does it increase performance for your use case?
• What do you think about the API? (e.g. do you think dedicated next_number_u64, ... would be better?)
• What do you think about the internal implementation?

Any feedback is appreciated! Ideally also with a bit of context on your use case.
This helps deciding whether to stabilize the method or how to refactor and improve it.