Enum/Product namespace

[leonardt]

We should either: (a) have a list of reserved names and error when a user provides an enum field that uses one (see https://github.com/phanrahan/magma/issues/500) (b) use a separate namespace for enum fields that avoids clobbering the internally used names by the type system, so the user can use arbitrary names without breaking internal code.

[splhack]

Product needs same thing #567

[cdonovick]

hwtypes uses a list of reserved names which may not be fields. There is also list of reserved _names_ which if set will have strange side effects but allow for extension.

https://github.com/leonardt/hwtypes/blob/master/hwtypes/adt_meta.py#L40 https://github.com/leonardt/hwtypes/blob/master/tests/test_adt.py#L359

It would make sense for magma to do something