mapper compilation of non_primary mappers is buggy

[Mike Bayer]

the compilation phase of mappers focuses on primary mappers. if your primary mapper setup contains a lot of references to "non_primary" mappers, all the old problems of mapper depedencies come up again since there is no global list of non_primary mappers to be compiled. need to either fix this, or create clearer exceptions when non_primary mappers are used in places that they shouldnt really be used (or determine exactly what uses are appropriate for non_primary mappers....).

[Mike Bayer]

compilation is getting simpler and the usage of non-primary mappers has decreased. going to let this one sit for awhile, although we should look into consolidating the "is_primary" and "non_primary" flags into a single clearer flag.

[Mike Bayer]

this has been fixed as of 0.4.2, all mappers are now present in the mapper_registry and get compiled as needed.