Factor out function to determine code for a field's value

We might need this separately later on.
Schema: Change an internal method name.
Schema: refactor get-dump-function-code to remove side effects.
Schema: supply namespace-dict to custom dump-function

instead of polluting the schema instance's namespace and relying on that
define dump_function in custom-made namespace

instead of providing namespace to function as a parameter
Schema: Move loop into generated dump-function.

This speeds up serializing of collections a little.
Change some comments.
schema: Update comments/docstrings.
schema: Change namespace def of generated dump function.

Add OrderedDict to namespace at the correct location (was in
Schema.__init__, now at method where rest of namespace is determined).
Update changelog.
Merge branch 'feature/refactor' into develop
Tests: PEP8 (where easily possible).