Add Display implementation

[joboet]

It would be useful to be able to print strings directly without converting them to a String, thereby avoiding the allocation.

While trying to implement this locally, I noticed that implementing Display automatically implies a ToString implementation, which conflicts with the current to_string functions as they return an error when encountering invalid code units, whereas my Display implementation performs the conversion lossily. Therefore, implementing Display would be a breaking change.

Solutions I could think of:

• Rename the conversion methods
  Pro: makes displaying strings convenient
  Con: breaks the API and does not follow the naming guidelines
• Add a display method similar to Path::display
  Pro: allows configuring different replacement options, like escaping invalid units
  Con: is not as straightforward and complicates usage when using proc-macros like thiserror

[yuandrew]

I agree that having a Display impl would be very useful.

I also think the default Debug impl should be swapped out for something more similar to the regular String debug implementation, instead of printing the underlying Vec as raw bytes.

[starkat99]

I've added display method to the strings. The new Display object by default does a replacement character for invalid values, with an {:#} alternate formatting option that entirely skips invalid values when printing. If you have better ideas for the alternate formatting, let me know, I couldn't think of anything else particularly useful off the top of my head.

I've also improved the Debug implementations. They now do standard escape_debug style strings on valid UTF data, just like std strings, but invalid values are displayed as \<XXXX> (XXXX being hex value), an intentionally invalid escape sequence that makes it relatively easy to see the raw invalid values.