Currently we have the class Schema for when you need to define more stuff on a field than just the type and default.

But this is a confusing name since not all it's arguments refer to the schema. It'll get more confusing if we want to add more arguments to Schema/Field, eg. a more complex alias system #477.

Therefore we should rename it to Field.

This would require:

• renaming the current Field class, perhaps renaming the entire fields.py module.
• Schema needs to carry on working for a while but with a deprecation warning

What does everyone thing?

@tiangolo Schema was your work originally, are you ok with this?