Break out const_addr_of.
Implement most of adjustment handling for consts.

Changes the ad-hoc closure adjustment into using adjustment info instead
of being separately driven from types, and likewise for autoderef.

Also takes care of autoref (the cases we should be seeing in consts,
at least, since we can't be doing method calls which would need the
ref-to-vec mode), which didn't quite work right previously.

However, "dereference" of a newtype isn't handled yet....
Let const_deref "dereference" newtypes.
Make a const translation test match TESTNAME=const
Don't print addr_of(addr_of(e)) as `&&e`, which means something else.
Add some tests for const auto-deref.

const-autoderef.rs is based on an example from jdm in IRC (but the
quadruple-pointer part is entirely my fault).
Don't copy const data to do an autoderef+autoref.
Const dereference works now, so allow it.
add a TotalOrd impl for the unit type
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 #5291 : pcwalton/rust/drop-lint, r=pcwalton

r? @nikomatsakis
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.
treemap: make set_advance public
treemap: indentation fixes
treemap: inline the TreeSet wrappers
treemap: add more tests for set difference
Add deriving_eq to Cell.
auto merge of #5318 : jdm/rust/deriving_cell, r=pcwalton

r? @pcwalton
core: Add vec::raw::mut_buf_as_slice