std: Remove ThreadPerCore spawn mode. Unused
Add dependence arc from running rustc to its libraries.

This commit fixes some oversights in the Makefile where rustc could be
invoked without some of its dependencies yet in place.  (I encountered
the problem in practice; its not just theoretical.)

As written in Makefile.in, $(STAGE$(1)_T_$(2)_H_$(3)) is the way one
writes an invocation of rustc where $(1) is the stage number $(2) is
the target triple $(3) is the host triple.  (Other uses of the macro
may plug in actual values or different parameters in for those three
formal parameters.)

When you have invocations of $(STAGE...), you need to make sure that
its dependences are satisfied; otherwise, if someone is using `make
-jN` for certain (large-ish) `N`, one can encounter situations where
GNU make attempts to invoke `rustc` before it has actually copied some
of its libraries into place, such as libmorestack.a, which causes a
link failure when the rustc invocation attempts to link in those
libraries.

In this case, the main prerequisite to add is TSREQ$(1)_T_$(2)_H_$(3),
which is described in Makefile.in as "Prerequisites for using the
stageN compiler to build target artifacts"

----

In addition to adding the extra dependences on TSREQ..., I also
replaced occurrences of the pattern:

    TSREQ$(1)_T_$(2)_H_$(3)
    $$(TLIB$(1)_T_$(2)_H_$(3))/$(CFG_STDLIB_$(2))
    $$(TLIB$(1)_T_$(2)_H_$(3))/$(CFG_EXTRALIB_$(2))

with:

    SREQ$(1)_T_$(2)_H_$(3)

which is equivalent to the above, as defined in Makefile.in

----

Finally, for the cases where TSREQ was missing in tests.mk, I went
ahead and put in a dependence on SREQ rather than just TSREQ, since it
was not clear to me how one could expect to compile those cases
without stdlib and extralib.

(It could well be that I should have gone ahead and done the same in
 other cases where I saw TSREQ was missing, and put SREQ in those
 cases as well.  But this seemed like a good measure for now, without
 needing to tax my understanding of the overall makefile
 infrastructure much further.)
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
Fix warnings in libextra tests

Most of these are "unneccesary allocation" in bitv, for ~[false, ..] instead
of [false, ..].
Fix warnings in libstd and librusti tests
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 `@`.
librustc: Remove a bunch of `@` boxes from `Match`.

This will be needed to add `'static` bounds to `@`.
librustc: Remove `&` inside `@` from borrowck/check_loans.
Start of data structure cleanup for trait system. Get rid of CoherenceInfo, make trait_impls less bogus.
Refactor some coherence/method trans code, add an impls map to tcx.

Rewrite method_with_name_or_default to use the new impls map.
Get rid of ProvidedMethodsMap.
Get rid of resolve::MethodInfo. Closes #4946.
Add provided method information to ty::Method. Get rid of ProvidedMethodSource.
Refactor a handful of stuff in the metadata encoder.
Export information about used default methods instead of regenerating it. Closes #7862.
librustc: Forbid `&` pointers (other than `&'static`) inside `@` boxes.

This makes custom borrowing implementations for custom smart pointers
sound.
rustpkg: Make rustpkg commands work without a package ID

`rustpkg build`, if executed in a package source directory inside
a workspace, will now build that package. By "inside a workspace"
I mean that the parent directory has to be called `src`, and rustpkg
will create a `build` directory in .. if there isn't already one.

Same goes for `rustpkg install` and `rustpkg clean`.

For the time being, `rustpkg build` (etc.) will still error out if
you run it inside a directory whose parent isn't called `src`.
I'm not sure whether or not it's desirable to have it do something
in a non-workspace directory.