Register snapshots
auto merge of #17159 : brson/rust/snaps, r=alexcrichton

This switches win64 hosts to bootstrap from win64 snaps.
Remove largely unused context from Visitor.
Track the visited AST's lifetime throughout Visitor.
auto merge of #17069 : eddyb/rust/visitor, r=pnkfelix

Few visitors used the context passing feature and it can be easily emulated.
The added lifetime threading allows a visitor to keep safe references to AST
nodes it visits, making a non-owning ast_map design possible, for #13316.
Update error messages in compile-fail tests
Take core as a dependency on librlibc. This is needed so that it can
see the lang-items for Sized etc. @acrichto and @thestinger had no
objections.
Guts of the new trait matching algorithm, not yet in use
Port coherence to use the new trait matching code
trans -- stop tracking vtables precisely, instead recompute as needed.
typeck/kind -- stop using old trait framework.

- Unify the "well-formedness" checking that typeck was already doing with what
  was taking place in kind.
- Move requirements that things be sized into typeck.
- I left the checking on upvars in kind, though I think it should eventually be
  refactored into regionck (which would perhaps be renamed).

This reflects a general plan to convert typeck so that it registers
obligations or other pending things for conditions it cannot check
eventually. This makes it easier to identify all the conditions that
apply to an AST expression, but can also influence inference in somec
cases (e.g., `Send` implies `'static`, so I already had to promote a lot
of the checking that `kind.rs` was doing into typeck, this branch just
continues the process).
misc ppaux changes
add Send bound on impl because stricter trait checking requires it
silence various warnings in stdlib, no idea why they suddenly started