Also, makes the pretty-printer use & instead of @ as much as possible,
which will help with later changes, though in the interim has produced
some... interesting constructs.

is very common, and the replacement (.iter().transform().collect()) is very
ugly.

'Borrow' stack closures rather than copying them (e.g., "|x|f(x)"), in prep for making them noncopyable.

They evaluated the receiver twice. They should be added back with
`AddAssign`, `SubAssign`, etc., traits.

Mostly just low-haning fruit, i.e. function arguments that were @ even
though & would work just as well.

Reduces librustc.so size by 200k when compiling without -O, by 100k when
compiling with -O.

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.

I removed the `static-method-test.rs` test because it was heavily based
on `BaseIter` and there are plenty of other more complex uses of static
methods anyway.