Implement vfree to release memory allocated by vmalloc

[salekinsirajus]

This function frees heap space (previously allocated with vmalloc). vfree returns OK in case of success, and SYSERR in case of failure.

syscall vfree (char* ptr, uint32 nbytes)

Your implementation should also handle the case where vfree is invoked incorrectly (i.e., on pages that had not been allocated before). In case of failure, none of the pages involved should be freed.

NOTE: what if nbytes is different from what was allocated? Do we also merge and split?