Fix help messages
Massive massive refactoring of SELECT and INFER.

Individual confidences are now supported in INFER queries with 'conf', e.g.
INFER a conf 0.4, b conf 0.7, c FROM table.

Where clause confidences are now supported for all where clause operators.

Huge speedup/refactoring in the order of function evaluations:
now, functions (including imputations) are only evaluated when necessary,
and are never computed more than once per query (e.g. if in the query clause,
where clause, and order by clause, the same value will be used for all).