update docs

Break out operators into common

minor

Allow `as` sexp for joins

Use uniform as syntax between select and from

Allow SELECT AS

Allow subqueries in from position

Validate against multiple AS

Explicitly mark column references without tables

simplify from

Break up from; simplify code walker

Make table binding optional

Minor reorg

Begin support for postgres

Add documentation of expressions API

Accept subqueries

implement fragments

Test and simplified sql rendering

Simple smoke testing around compose and select

unit tests for `from`