[Second review] SE-0390: Noncopyable structs and enums

[Joe_Groff]

@ksluder

Absolutely. I personally predict I will use move-only types most frequently as a wrapper for copyable types, as part of guaranteeing an implementation of a data structure avoids copies. Wrapping will involve consuming the copyable value.

You should never need to do this, except maybe in the extreme short term, since we plan to make it so that borrow bindings are not implicitly copyable even when bound to values of copyable type.