Unify and nest structs and enums

Another alternative to RFC rust-lang#5 and an extension/variant of RFC rust-lang#11.

Unify enums and structs by allowing enums to have fields, and structs to have
variants. Allow nested enums/structs. Virtual dispatch of methods on struct/enum
pointers. Remove struct variants. Treat enum variants as first class. Possibly
remove nullary structs and tuple structs.

0000-enum-struct.md

@@ -0,0 +1,25 @@
+- Start Date: (fill me in with today's date, YYY-MM-DD)
+- RFC PR #: (leave this empty)
+- Rust Issue #: (leave this empty)
+
+# Summary
+
+One para explanation of the feature.
+
+# Motivation
+
+Why are we doing this? What use cases does it support? What is the expected outcome?
+
+# Detailed design
+
+This is the bulk of the RFC. Explain the design in enough detail for somebody familiar
+with the language to understand, and for somebody familiar with the compiler to implement.
+This should get into specifics and corner-cases, and include examples of how the feature is used.
+
+# Alternatives
+
+What other designs have been considered? What is the impact of not doing this?
+
+# Unresolved questions
+
+What parts of the design are still TBD?