Pass FPSmanager size as size_t to malloc instead of u64.

Changes the FPSmanager constructor to use the architecture dependent
size_t type from the libc crate instead of u64 as size when allocating
the memory for the instance.
Merge pull request #2 from phaylon/fspm-size_t-to-malloc

Pass FPSmanager size as size_t to malloc instead of u64.