Is it possible to prevent the heap from being compacted, or pin
individual objects?

Suppose I write a C function C_func, which takes a string argument.
It passes that argument to some library function lib_func (which has
been written by a third party).  bar invokes a C callback C_cb.  In my
wrapper for Caml, C_cb invokes the Caml function cb.  cb conses and
triggers heap compaction.  After return from cb and C_cb, bar uses the
old address of the string, which is incorrect.

Currently, I copy all strings used in such a scenario, but this adds
quite a bit of overhead.