repairs for `srcloc->string`

Fix predicate on argument, including allowing chaperoned srclocs.

Closes PR 14376
readline: fix broken registration of "mzscheme" as name

Maybe it should be "racket", instead, but leaving as "mzscheme"
in case anyone has customizations using that name.
[unsafe-]fl{real,image}-part: fix contracts in docs and errors

Closes PR 14368