Simplify struct representation.

Out goes the extra layer of struct wrapping; the destructedness flag is
added to the end of the struct.  This means that, if the struct
previously had alignment padding at the end, the flag will live there
instead of increasing the struct size.
Get rid of the `Unit` enum representation.

The only thing we really lose is that C-like enums with one variant and a
non-zero discriminant now take up space, but I do not think this is a
common usage.  As previously noted, that was mostly there for
transitional compatibility with the pre-adt.rs codebase.
auto merge of #5314 : jld/rust/adt-simplification, r=pcwalton

Struct and enum representations have some complicatedness that's no longer needed.  Now that everything's in one place and has access to anything we'd want to know about the type, flatten some of that out.  Slight changes to representations in some cases.