add a region-based alt& statement

[nikomatsakis]

alt today creates implicit references. We should make a temporary alt& that creates regions for bindings, deprecate alt, and gradually perform the migration.

[bblum]

I support this. I've been writing a doubly-linked list today and several times have fought "error: illegal borrow unless pure: enum variant in aliasable, mutable location". Very unfriendly.

[bblum]

#3271 subsumes this.