Add `as_bytes()` for `FromUtf8Error`.

Add `as_bytes()` for `FromUtf8Error`.

This change allows to obtain an underlying invalid UTF-8 bytes
without `FromUtf8Error` destruction. Such method may be useful
for example in a library that attempts to save both valid and
invalid UTF-8 strings in some struct and to be able to provide
immutable access to it without destruction.