It is much more convenient to skip source creation.
Also fix some bugs:
1. `cryptoSource.Int63()` now returns number in [0, 1<<63) as required
by `rand.Source` interface.
2. Replace `cryptoSource.Uint63()` with `cryptoSource.Uint64` to allow
generate uint64 numbers directly (see rand.Source64 docs).
Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>
