- Query.select_from() now replaces all existing FROM criterion with

- Query.select_from() now replaces all existing FROM criterion with
the given argument; the previous behavior of constructing a list
of FROM clauses was generally not useful as is required
filter() calls to create join criterion, and new tables introduced
within filter() already add themselves to the FROM clause.  The
new behavior allows not just joins from the main table, but select
statements as well.  Filter criterion, order bys, eager load
clauses will be "aliased" against the given statement.