add group by field + func
max
changes
unit tests.
test things; simplify things; succeed.
simplify things; don't hack around nested MakeArrays for params to without/pluck/hasfields
simplify UpdateByFunc/Obj.
factor out common table update method.
remove unused MapBase class.
add maybe_map util.
simplify MergePoly
simplify joins
simplify HasFields
move things around.
simplify get + getall
simplify Replace.