std: Work around some failing 'run' tests when valgrinding. #7224

Under valgrind on 64->32 cross compiles the dynamic linker is emitting
some error messages on stderr, which interferes with the tests that
are checking stderr.
rusti: Disable tests on macos harder
std::rt: Work around a dynamic borrowck bug
rustc: Dispose of LLVM passes in test cases
libsyntax: Remove `drop` as a keyword.
Fix update snapshot script for rename
Ignore rustpkg tests
core -> extra for mac
xfail box-related debug-info tests r=snapshot
ignore inherited environment test under valgrind r=snapshot
_Actually_ xfail debuginfo tests
Remove all #[cfg(stage0)]-protected code

New snapshot means this can all go. Also removes places that have
comments that say they are workarounds for stage0 errors.
libstd: cleanup warnings
libsyntax: cleanup warnings
libextra: cleanup warnings
librust: cleanup warnings (except 1)
Update snapshots.txt
vec: remove each_const

An Iterator implementation can be made for &const [T] if it turns out
to be necessary for some use case.
vec: remove eachi

replaced by the `enumerate` method from std::iterator
vec: rm old_iter implementations, except BaseIter

The removed test for issue #2611 is well covered by the `std::iterator`
module itself.

This adds the `count` method to `IteratorUtil` to replace `EqIter`.