Experimental NumberFormatter for JSON writer

[Marcono1234]

Struson 0.8 will include a new experimental trait NumberFormatter. This trait is intended for customizing how JsonStreamWriter formats numbers (u8, f64, ...) as JSON number strings.

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

• Do you like or don't like the new functionality, why?
• What kinds of custom number formatters are you implementing?
• Are you using the default formatter, but the introduction of the customization somehow breaks your existing code or requires you to redundantly specify types?
• What do you think about the API?
  • trait NumberFormatter
  • Specifically the method NumberFormatter::format_number_str
    Do you think that method is useful?
  • method JsonWriter::number_formatter
    In case you are implementing a custom JsonWriter, does that method make sense for your use case?
    (note that currently Struson's Serde integration uses it when serializing numbers as map keys)
• 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 this API or how to refactor and improve it.

[Marcono1234]

An alternative would be something more low-level, like serde_json's Formatter trait, which not only covers number formatting but formatting of the whole JSON document.

However, that might be too low-level, covering things which the user should not be able to customize, e.g. how a JSON null is written. It would also be cumbersome for users who want to customize only number formatting but then have to implement a bunch of other unrelated methods as well (unless they have a default impl).
Instead something like a dedicated 'pretty printer' trait which only customizes whitespace usage (and not number formatting) might be more appropriate (see also the Struson pretty-printer branch).