Suggest correct path in include_bytes!

[kornelski]

include_bytes! paths are relative, and I'm often not sure how nested is the .rs file that I'm editing, so I have to guess the number of "../..". This change searches .. and ../.. for the given file and offers corrected path as a suggestion.

I wasn't sure how to get the right span, and how to properly escape it.

error: couldn't read src/file.txt: No such file or directory (os error 2)
 --> src/main.rs:2:13
  |
2 |     let x = include_bytes!("file.txt");
  |             ^^^^^^^^^^^^^^^----------^
  |                            |
  |                            help: it's in a parent directory: `"../../file.txt"`