Have PgSelectStep leverage unary steps

Relates to #1973; PgSelectStep and related classes should be able to `addUnaryDependency` for first/etc and thus not need to do such complex SQL for first/last/cursor/etc. Should be able to use this to eliminate some `eval` calls also?

Start with PgSelectStep and the related classes, then read the discussion in #1973. Trace how first/last/cursor operations currently build SQL and where eval calls are used. Done means unary dependencies can handle these operations and the unnecessary SQL complexity and eval calls are removed.

Written by the indexing model from the issue text.