Register new snapshots

Understand 'pkgid' in stage0. As a bonus, the snapshot now contains now metadata
(now that those changes have landed), and the snapshot download is half as large
as it used to be!
auto merge of #10919 : alexcrichton/rust/snapshots, r=brson

Understand 'pkgid' in stage0. As a bonus, the snapshot now contains now metadata
(now that those changes have landed), and the snapshot download is half as large
as it used to be!
AST Visitor now walks enum discriminant expression
Mark live codes in struct/enum for dead-code pass
auto merge of #10994 : ktt3ja/rust/issue-10956, r=alexcrichton

Types used inside live struct or enum are now marked live.

Fix #10956 and #10993.
debuginfo: Always set column information to zero for source locations.
debuginfo: Clear debug source locations at beginning of functions.
debuginfo: Clear source positions where they'd confuse LLVM otherwise.
debuginfo: Create separate lexical block for function bodies.
debuginfo: Get rid of redundant argument copying with debuginfo activated
debuginfo: Use same behavior as Clang for calculating scope_line parameter when creating function DIEs.
debuginfo: Set the is_local_to_unit attribute of functions correctly
debuginfo: Set correct source position for function calls.
debuginfo: Add test cases for function prologue handling.
debuginfo: Added some documentation of source location management.
debuginfo: Add comment on is_local_to_unit parameter.
vec: make the move iterator fast for all types

Closes #10976
auto merge of #10995 : thestinger/rust/fast_move_iter, r=huonw

Closes #10976

Taken from https://github.com/thestinger/rust-core/commit/863fbaaa125f1bb290e28cbf22236406cf0aebcf, so it will be easily updated to the better vector representation too.
auto merge of #10966 : michaelwoerister/rust/prelude2, r=cmr

This PR improves the stepping experience in GDB. It contains some fine tuning of line information and makes *rustc* produce nearly the same IR/DWARF as Clang. The focus of the changes is function prologue handling which has caused some problems in the past (https://github.com/mozilla/rust/issues/9641).

It seems that GDB does not properly handle function prologues when the function uses segmented stacks, i.e. it does not recognize that the `__morestack` check is part of the prologue. When setting a breakpoint like `break foo` it will set the break point before the arguments of `foo()` have been loaded and still contain bogus values. For function with the #[no_split_stack] attribute this problem has never occurred for me so I'm pretty sure that segmented stacks are the cause of the problem. @jdm mentioned that segmented stack won't be completely abandoned after all. I'd be grateful if you could tell me about what the future might bring in this regard (@brson, @cmr).

Anyway, this PR should alleviate this problem at least in the case when setting breakpoints using line numbers and also make it less confusing when setting them via function names because then GDB will break *before* the first statement where one could conceivably argue that arguments need not be initialized yet.

Also, a koala: :koala:

Cheers,
Michael
Rewrite std::comm

* Streams are now ~3x faster than before (fewer allocations and more optimized)
    * Based on a single-producer single-consumer lock-free queue that doesn't
      always have to allocate on every send.
    * Blocking via mutexes/cond vars outside the runtime
* Streams work in/out of the runtime seamlessly
* Select now works in/out of the runtime seamlessly
* Streams will now fail!() on send() if the other end has hung up
    * try_send() will not fail
* PortOne/ChanOne removed
* SharedPort removed
* MegaPipe removed
* Generic select removed (only one kind of port now)
* API redesign
    * try_recv == never block
    * recv_opt == block, don't fail
    * iter() == Iterator<T> for Port<T>
    * removed peek
    * Type::new
* Removed rt::comm