Rework the compiler to use table aliases, subqueries, limit, offset and order by

[milessabin]

Currently the SQL compiler targets a very limited subset of SQL: left joins and where clauses with simple, non-subquery, expressions. The staging/batching mechanism mitigates the lack of table aliasing somewhat but isn't ideal. In some cases this has led to query algebra constructs being squeezed into the current translation inappropriately (see #119 for a consequence of handling what should be a subquery as a join).

The compiler needs to be reworked in at least the following ways,

• recursive queries should be compiled in terms of table aliases
• projected predicates should compile to subqueries
• interfaces and unions should be compiled in terms sql unions
• limit and order by should be compiled to the natural sql equivalents rather then being implemented programmatically in the interpreter.

[milessabin]

Still some work to do on interfaces and unions, but otherwise I'm declaring victory as of #152.