reduce CGO overhead for passing values
No pointer optimisation txn (#142) * added valNP (no pointer value) argument to Txn obj tp increase performance (see GetFast for more details) * replace txn.Get with txn.GetFast realization * change type of txn.val from *C.MDBX_val to C.MDBX_val * no longer takes pointer from env val --------- Co-authored-by: Ilya Miheev <[email protected]>