defthing: use #:id expression instead of 'id'

[bennn]

Change implementation of defthing so that if the user gives an id-expr via the #:id keyword, then defthing never uses the default id.

I think this is right, based on the docs. But maybe the docs should change.

cc @dedbox for reporting this

---

Example program:

#lang scribble/manual

@defthing[#:id (datum->syntax #f 'foo) dont-use integer?]{
  Something
}

Before this PR:

After this PR:

[bennn]

e2ea830