Propose some plans in the playground

Here I propose syntax for being able to implement Clone on enums, etc.

The branch contains, all in the playground/

• Two new examples clone-full.rs and ref-version.rs which represent things I am intending to work on getting working soon.
• Deprecation of hash2.rs and clonelike.rs
• Two fanciful examples of things I thought in passing.

@nickm I'd appreciate it if you'd have a quick look at clone-full and ref-version. In particular, possibly controversial proposals, compared to clonelike and hash2:

• Take the syntax $[ ... ] for pasting. I had considered $< .... > and the previous suggestion was [< ... >].
• $fpatname for "name of the binding that $vpat made for this field", rather than $pfname.
• Funky new expansions ${tvariants } and ${vdefine } for conditionally emitting things that enums need, without the use of ${if is_enum ...}
• Funky new expansions ${fspec } for conditionally emitting things that struct field definitions need, without the use of ${if is_struct_variant_or_struct ...}