Consolidate raw representations of rust values

[alexcrichton]

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, unwrap, 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).

This is progress on #6790. I tried to get a nice interface for a trait to implement in the raw module, but I was unable to come up with one. The hard part is that there are so many different directions to go from one way to another that it's difficult to find a pattern to follow to implement a trait with. Someone else might have some better luck though.

[brson]

Looks like some good cleanups. 👍

[alexcrichton]

@brson, @huonw, now it's

trait Repr { fn repr() }

[alexcrichton]

@huonw pointed out that std::repr::Repr also exists, so for the sake of less confusion I've renamed the trait to his suggestion of RawRepr

r? @brson (just want to make sure this is ok with you)

[alexcrichton]

Actually, after thinking about this some more, I like Repr better because that's why we have package namespaces. raw::Repr is different enough from repr::Repr that I prefer the shorter name.