Allow merge when empty left case.
Divide context module from join module.

--HG--
rename : relational-join/src/Database/Relational/Query/Join.hs => relational-join/src/Database/Relational/Query/Internal/Context.hs
Renamed.
Preparing to change product structure.
Update interfaces of Product module.
Preparing to change product structure. Use growLeft interface.
Change product structure.