Size attribute is ambiguous

[treiher]

The 'Size attribute can be used to refer to the size of a composite field as well as to the size of a message field (since #384). As field and type names are not necessarily unique, this can lead to conflicts:

package Test is

   type T is mod 2**8;

   type M is
      message
         A : T;
         T : Opaque
            with Size => 16;
      end message;

end Test;

model: error: contradicting condition in "Test::M"
test.rflx:7:10: model: info: on path: "A"
test.rflx:8:10: model: info: on path: "T"
test.rflx:9:26: model: info: unsatisfied "T'Size = 16"
model: info: unsatisfied "T'Size = 8"

Changes

1. Only allow using fully-qualified type names to prevent such conflicts (e.g., Test::T).
2. Allow using 'Size attribute for all message fields regardless of field type.