libcore: Deprecate advance method on Iterators.
libcollections: Use iterators instead of old-style loops.
libgetopts: Use iterators instead of old-style loops.
libsyntax: Remove uses of advance.
librustc: Remove uses of advance.
tests: Remove uses of advance.
auto merge of #15514 : luqmana/rust/die-advance-die, r=alexcrichton

Closes #15492.