Only rename identifiers that are textually equal

[sorawee]

It is possible to have an arrow between two identifiers that are not
textually equivalent (definitely possible in DrRacket, and theoretically
possible in Racket Mode).

In the above event, renaming one identifier should not change the other.

See also racket/drracket#415 for a similar fix in DrRacket, and
racket/racket#3391 which would start to make Racket Mode behave
incorrectly. In particular, with the above PR, renaming
the identifier a in the following program should not rename
all-defined-out as well.

#lang racket
(provide (all-defined-out))
(define a 1)

[sorawee]

Note that with the above mentioned PR, attempting to run racket-xp-rename at all-defined-out in

#lang racket
(provide (all-defined-out))
(define a 1)
(define b 1)

will offer users to rename a random defined identifier (it's b for me). While this doesn't seem ideal, the problematic behavior that I attempt to fix with this PR (proceeding to rename it should not rename all-defined-out) is indeed repaired. So I don't consider it to be in the scope of this current PR.