There's no point in having vinyl_engine and vinyl_index wrapper structs
to bind vy_env and vy_lsm to struct engine and index. Instead we can
simply embed engine and index in vy_env and vy_lsm. This will simplify
further development, e.g. this will allow us to move reference counting
from vy_lsm up to struct index so that it can be used in the generic
code.