This replaces the Value struct by an interface to allow us to override some
values behavior in the future.
It also introduces a new package types, which contains type definitions, comparison,
and arithmetics.
Concerning encoding, Genji now only uses on type of encoding for values. This simplifies
indexing logic as well as table access in general.
Previously, expressions and params were evaluated
during the planning phase. This change builds the query
plan without evaluating params and expressions
which are then evaluated only during the execution phase.
