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.
This was used to buffer parsed expressions to be used as keys in the
projected documents.
This is no longer necessary, as every expression can now be stringified.
