hdf5: fix []C.hid_t <-> []uint type prunning

[sbinet]

there are many places where we do:

func f(vs []uint) {
    cvs := (*C.hid_t)(unsafe.Pointer(&vs[0]))
    C.cfunc(C.int(len(vs)), cvs)
}

(or equivalent.)

either replace all these type prunning snippets or insert a func init() that checks this type prunning is actually working for the platform/OS/compiler setup at hand.