perf tweaks

perf tweaks

- avoid abc checks in distill_20
- ColumnEntity subclasses are unique to their compile state and
have no querycontext specific state.  They can do a simple memoize of their
fetch_column without using attributes, and they can memoize their
_getter() too so that it goes into the cache, just like
instance_processor() does.
- unify ORMColumnEntity and RawColumnEntity for the row processor part,
add some test coverage for the case where it is used in a from_statement
- do a faster generate if there are no memoized entries
- query._params is always immutabledict
Change-Id: I1e2dfe607a1749b5b434fc11f9348ee631501dfa