This moves the raw struct layout of closures, vectors, boxes, and strings into a
new `unstable::raw` module. This is meant to be a centralized location to find
information for the layout of these values.

As safe method, `repr`, is provided to convert a rust value to its raw
representation. Unsafe methods to convert back are not provided because they are
rarely used and too numerous to write an implementation for each (not much of a
common pattern).

To achieve this, the following changes were made:
* Move TyDesc, TyVisitor and Opaque to std::unstable::intrinsics
* Convert TyDesc, TyVisitor and Opaque to lang items instead of specially
  handling the intrinsics module
* Removed TypeDesc, FreeGlue and get_type_desc() from sys

Fixes #3475.

Adds documentation for various things that I understand.
Adds #[allow(missing_doc)] for lots of things that I don't understand.

This only changes the directory names; it does not change the "real"
metadata names.

bare function store (which is not in fact a kind of value) but rather
ty::TraitRef.  Removes many uses of fail!() and other telltale signs of
type-semantic mismatch.

cc #4183 (not a fix, but related)

correct handling of moves for struct-record update.

Part of #3678.  Fixes #2828, #3904, #4719.

librustc: Terminate name searches at the nearest module scope for paths that contain at least two components. r=graydon

Sadly, there's only one file that requires deprecated modes
(stackwalk)... So, forbid them everywhere else.

This removes most explicit uses of the + argument mode. Pending a
snapshot, I had to remove the forbid(deprecated_modes) pragma from
a bunch of files. I'll put it back!

+ mode still has to be used in a few places for functions that get
moved (see task.rs)

The changes outside core and std are due to the to_bytes trait and
making the compiler (with legacy modes on) agree with the libraries
(with legacy modes off) about modes.