add FromUtf8Error in sync with std

[johann-cm]

I'm proposing changing the error type of String::from_utf8 form core::str::Utf8Error to heapless::string::FromUtf8Error. This would mirror the error type of std::String::from_utf8 (which is std::string::FromUtf8Error). This is a breaking change.

FromUtf8Error has the advantage that you can regain ownership over the data that you want to convert to UTF-8 in case of an error, which you currently lose in that case.
The change would have made it possible for me in #639 to implement into_string without unsafe code.

[sgued]

Thank you for the PR!

I'm not convinced it is that useful

The ergonomics of having the Error type have a const-generic and not being Copy don't seem justified. Unlike in std, we never allocate, so cloning the Vec is somewhat cheap and can be done before calling the from_utf8 function without a significant performance cost. I suspect LLVM is very capable of avoiding the copy when doing this. If a benchmark shows this is justified, I would merge this, but otherwise I think keeping it the way it currently is is best from a downstream ergonomics standpoint.