Also, register string memory as additional GC pressure.
The subprocess module repeatedly allocates a 1MB str as a buffer
for reading from file descriptors.  The first issue was that we would
never free this memory; but even once we would, we wouldn't trigger a GC
since there were only small GC allocations (sizeof(BoxedString) at a time)
but with large other memory pressure, so add a hook for adding GC pressure.