libsyntax: Add `parse_view_item` method to Parser
libsyntax: Add tests for `parse_view_item`
Move trans, back, driver, and back into a new crate, rustc_trans. Reduces memory usage significantly and opens opportunities for more parallel compilation.
make cloned generic over deref... and have its tests actually run
add Cloned iterator adaptor
auto merge of #19070 : nikomatsakis/rust/crates, r=nikomatsakis

Reduces memory usage significantly and opens opportunities for more parallel compilation.

This PR was previously #19002 but I closed it because bors didn't seem to recognize the `r+` annotations there.
auto merge of #19060 : Gankro/rust/super-cloned, r=aturon

Edit: whoops, didn't mean to hit post.

Anyway, this is something I tried to do when I first implemented cloned, but couldn't figure out. Somewhere between then and the PR actually landing, we got Deref of references, so now this works! :tada: 

Also turns out the test for the functionality was never marked as a #[test]. Oops!

Also added a Cloned iterator adaptor. If this isn't desirable, it can be taken out of the PR (seperate commits).
Place parenthetical notation under the `unboxed_closure` feature-gate.
Consolidate the `unboxed_closure_sugar` and `unboxed_closure` feature gates.
Pull out the fn-sig sub/lub/glb code and generalize it into a `higher_ranked` module. Also moves the docs to a more suitable place.
Switch the code to use De Bruijn indices rather than binder-ids.
Introduce some sanity checking assertions in trans, erase regions more aggressively.
Make it an error to use parenthetical notation with
something other than a trait type.
Convert TyPolyTraitRef to accept arbitary bounds, so that things like
`Box<for<'a> Foo<&'a T> + 'a>` can be accepted. Also cleanup the visitor/fold
in general, exposing more callbacks.
Stop checking the correctness of explicit self twice; instead, just
use simple pattern matching to take a guess at what the method's self
category is in astconv, and check it more thoroughly later.
Miscellaneous reformatttings and renamings.
Test resolving of names with `for` binders.
Introduce machinery for higher-ranked TraitRefs
Allow impl's to have late-bound regions. Introduces another level of
region binding at the impl site, so for method types that come from impls,
it is necessary to liberate/instantiate late-bound regions at multiple
depths.
Update tests for `()` notation to use traits not structs
Update test for equivalency to include region binders in object types, add new tests relating to HRTB, consolidate the `unboxed_closures` and `overloaded_calls` feature gates.