Adding new ffi calls into the Hpc rts subsystem

Adding new ffi calls into the Hpc rts subsystem

foreign import ccall unsafe hs_hpc_write :: CString -> IO ()
foreign import ccall unsafe hs_hpc_read  :: CString -> IO ()

These write a Hpc description of the state of the world to a file,
or read a description into the current Hpc tickbox subsystem.