Declare enums as LLVM named structs

[brson]

Our new GC infrastructure will need to distinguish enum types so we should name them. They will still just be structs of i8s.

[nikomatsakis]

Can we please fix their representation as well at the same? Now that we've
monomorphized this should be easy. Actually, I even have the code lying
around in some repo or other---what stymied me was the dynamic case. I
guess I ought to dust it off and bring it in.

[brson]

This seems to require coming up with a canonical name for enums defined in external crates. Is there any other feature that does this in trans? @nikomatsakis I know you ran into a related issue during, I think, printing paths in error messages.

[brson]

I think I will just use the full path to the enum, from the perspective of whichever crate it is defined in, and tack on :: plus the crate number. Sounds unique to me.

[nikomatsakis]

I never came up with a good solution.

[brson]

Fixed by 6e909e3. Addressing alignment in #1645.

[brson]

Reverted because it broke rust-zmq

[brson]

Fixed by df0ef52