A few cosmetic changes.

Rename the `Memory' constructor that constructs MemoryVoucherDatabase
into `MemoryDB' to avoid conflict.

Also expose all functions for now, for testing via "stack ghci".

Use this function to check if the given voucher is unpaid. Perhaps
this can be done in Sql itself without resorting to set operations.

Also change a Maybe to an Either so we can stop printing an error inside the
ristretto code and just propagate it up to the caller.

None of the FFI values escape from `ristretto` so all of the FFI IO should be
safe to do with `unsafePerformIO`.

It's total amateur hour but that's what I am so who is surprised?

Therefore this approach is preferable