Remove copy from Display implementation

[ThePerkinrex]

Removes possible allocation, removing to_string.

Maybe this is not allowed, as it is a possible breaking change, as every T: Display implements ToString but not the other way around.

I thought it may be positive to note that the old implementation allocated when not needed, like when LocatedSpan<&str> is used

[progval]

Hmm, interesting. Sadly, https://github.com/rust-lang/rust/issues/31844 isn't stable yet, so we can't have both; and adding a cfg feature seems overkill (and doesn't really solve the problem since both can't be used in the same application).

Do you know any type that implements ToString but not Display?

[ThePerkinrex]

Not in std, but anyone could create it, in a dependent crate, for example.

That would mean that if somebody did that, which isn't recommended, and I think clippy advises against it, but it is possible, they would have a breaking change when updating.

Anyways, this isn't much, as I think not many people would use display on this type directly, but it could be something to note, to add it to the next breaking release (v4.0.0)

I just wanted to let you know of this