Add some documentaion to SqlExpr.
Re-order exports related to insertSelect.
Move public insertSelect API to Language module.
Thread IdentState through subqueries (fixes #28).

There used to be name clashes if a subquery referenced
an entity that was already being used on the outer query.
Now we thread the outer query's IdentState to its subqueries,
which use it instead of initialIdentState.

Note that clashes still may occur between subqueries of
a query, but I think that's harmless.
Bump version to 1.3.4.