Pre-compute dump-only and load-only fields; clean up signatures (#1328)
Pre-compute dump-only and load-only fields; clean up signatures (#1328)

* Pre-compute dump-only and load-only fields; clean up signatures

This results in a very small perf gain and removes
unnecessary parameters from _serialize, _deserialize, and _run_validator.

* Maintain field order on python 3.5

* Remove unnecessary accessor param

* Remove index_errors param

mwstobo/marshmallow,marshmallow-code/marshmallow