rustpkg: Eliminate a copy
Makefiles: make rustpkgtest depend on rustpkg executable
std: Fix perf of local allocations in newsched

Mostly optimizing TLS accesses to bring local heap allocation performance
closer to that of oldsched. It's not completely at parity but removing the
branches involved in supporting oldsched and optimizing pthread_get/setspecific
to instead use our dedicated TCB slot will probably make up for it.
auto merge of #8415 : brson/rust/newrt-local-heap-perf, r=pcwalton,brson

Mostly optimizing TLS accesses to bring local heap allocation performance
closer to that of oldsched. It's not completely at parity but removing the
branches involved in supporting oldsched and optimizing pthread_get/setspecific
to instead use our dedicated TCB slot will probably make up for it.
std/rustc/rustpkg/syntax: Support the `extern mod = ...` form

This commit allows you to write:

 extern mod x = "a/b/c";

which means rustc will search in the RUST_PATH for a package with
ID a/b/c, and bind it to the name `x` if it's found.

Incidentally, move get_relative_to from back::rpath into std::path
rustpkg: Simplify the PkgId struct

Get rid of special cases for names beginning with "rust-" or
containing hyphens, and just store a Path in a package ID. The Rust-identifier
for the crate is none of rustpkg's business.
auto merge of #8176 : catamorphism/rust/rustpkg-extern-mod, r=catamorphism

r? @graydon Also, notably, make rustpkgtest depend on the rustpkg executable (otherwise, tests that shell out to rustpgk might run when rustpkg doesn't exist).
Remove the C++ runtime. Sayonara
auto merge of #8387 : brson/rust/nooldrt, r=brson
Forbid pub/priv where it has no effect

Closes #5495