to_string and as_str have different implementations for Pair, which is pretty confusing and misleading

[janTatesa]

Describe the bug
When you do pair.to_string() and pair.as_str().to_string() you get different strings - the former gives the pair name and it's range, the latter the actual span content . This is pretty confusing as foo.as_str().to_string() should generally be equivalent to foo.to_string(). Users the library will think it is and it will lead to bugs. I personally did so and it wasn't pleasant to realize what was the cause

Expected behavior
pair.to_string() should produce the actual span content. There could be a different method for displaying the span name and range

[Teolhyn]

I could have a look at this. @tomtau if I fix the to_string() implementation so that it returns the span content, should I add another method for the functionality it provides currently?

E: I looked into it and as the to_string() comes from the Display impl and there is also the Debug implementation that gives the same output that to_string() is giving now.