compiler: add support for multirow inserts

compiler: add support for multirow inserts

Some databases support this syntax for inserts:

INSERT INTO table (id, name) VALUES
('v1', 'v2'),
('v3', 'v4');

which greatly increases INSERT speed.

It is now possible to pass a list of lists/tuples/dictionaries as
the values param to the Insert construct. We convert it to a flat
dictionary so we can continue using bind params. The above query
will be converted to:

INSERT INTO table (id, name) VALUES
(:id, :name),
(:id0, :name0);

Currently only supported on postgresql, mysql and sqlite.