The old `cast' didn't work right for a mismatch between
a pointer GCableness and the source or target types, and
it didn't work right for an GCable pointer with a non-zero
offset. While those pitfalls were documented, the first
of them definitely has been a source of bugs in code that
I wrote.
Also added `cpointer-gcable?'
* The old function was removed completely, people will get it from
`racket/base' anyway.
* I also removed its documentation. I thought about leaving a note in,
but if `define-ffi-definer' is the preferred style, then this should
be done when there's a way to make `define-ffi-definer' use it. (Eg,
some new #:keyword that adds a way to change the defined name.)
* Note that the function is added to `racket/private/string' and not to
`racket/string' because the latter deals only with strings, and the
new function accepts byte strings too. It might be better to start a
new `racket/regexp' module for these functions.
