This changes the OM dialect Class/ExternClass to use a return style
field name/type specifier syntax and, for non-extern class, a terminator
op in the style of hw.output.

The ClassLike code requires updating their parser/printer for the new
syntax.

This change introduces two new inherent attributes for ClassLike:
* fieldNames: array of string attribute corresponding to field names in
  definition order
* fieldTypes: dictionary mapping field names to types

The ClassLike code now provides an API to get the type of a field based
on its name, abstracting the underlying attribute storage.

The firrtl LowerClasses logic requires changes to use this API when
constructing OM classes, as well as in the dialect conversion logic to
handle the convering the stored types.

The OM evaluator similarly is updated for the new field API.

The OM linker is updated to skip constructing the field name to type map
and instead using the getFieldType API provided by the classOp.

The OM object field verifier also no longer needs to construct the field
name to type maps and instead can just use the new API.

Quite a few tests required updating for the new field syntax.

This reverts commit 38549a0.

This reverts commit 9226a5d.