add fdb_raw, which works with binaries, fix key-selector problem for range-begin

adjust subspace to work with binary prefixes
adjust fdb to be a tuple/term layer on top of fdb_raw
add previous/next functionality
make (most of) tests pass reliably, move (most of) tests to <<"__test">> subspace

moving tests to subspace has the benefit of not interfering with actual database

I still consider the fdb (tuple/term) layer dangerous, because it cannot be used
with a prefixed keys. Don't run fdb_test on database with valuable data.

Out of despair, I created a fdb_raw:init_and_open_try_5_times/1 .
The database doesn't open reliably. Don't know why.
add ranked set layer, tests pass