schema: Refactor some internals.

To better reflect the planned implementation of maintaining field order
SchemaMeta: Replace Schema namespace dict with OrderedDict

to later get the field order correct.
schema: Update helper functions to return OrderedDicts.

also: improve their docstrings a little.
schema: Implement ordered fields (plus tests).

Details are documented in docstrings.
Merge branch 'feature/ordered-fields' into develop
Document new field definition options.
schema: Fix typo in docstring.
docs: Add reminder: exclude/only mutually exclusive.
docs: Add comparison to marshmallow.
docs: Add anchor to validation-section, remove paragraph.