rt: Diagnose bad alloc index in release_alloc with RUSTRT_TRACK_ALLOCATIONS
rt: Print alloc backtraces for bad release_alloc with RUSTRT_TRACK_ALLOCATIONS

Probably the env var should be renamed from DETAILED_LEAKS but I'm leaving
aside that breaking change for now.
testsuite: Add compile-fail test for #5883
testsuite: Test for #6153, closes #6153
remove non-portable sed invocation as brson says we no longer have clang in tree.
librustc: Remove a bunch of `@` boxes from `Match`.

This will be needed to add `'static` bounds to `@`.
auto merge of #7859 : kmcallister/rust/rt-diag-messages, r=pcwalton

I added these while tracking down heap corruption in Servo.
auto merge of #7863 : catamorphism/rust/issue-5883, r=catamorphism
auto merge of #7868 : pnkfelix/rust/issue6929-remove-bogus-sed-invoke-from-configure, r=brson

... in tree.

Fix #6929
auto merge of #7871 : pcwalton/rust/de-at-match, r=brson

This will be needed to add `'static` bounds to `@`.

r? @nikomatsakis
Allow linking against crates with #[no_std]

Previously having optional lang_items caused an assertion failure at
compile-time, and then once that was fixed there was a segfault at runtime of
using a NULL crate-map (crates with no_std)