Document for-loop in tutorial section on loops
Revised Crate section of tutorial for clarity.
 Explain container iteration in the loop tutorial

 As suggested by @pnkfelix in #12161, this extends the examples by a for-loop
 that iterates over a string as an example for container iteration.
Fix broken link to the container guide
remove dead link to rustpkg documentation
Tweak test name and make it more specific
Add ignore-cross-compile directive for compiletest

Loadable syntax extensions don't work when cross compiling (see #12102), so the
fourcc tests all need to be ignored. They're valuable tests, so they shouldn't
be outright ignored, so they're now flagged with ignore-cross-compile
auto merge of #12171 : chromatic/rust/fix_crate_tutorial_typos, r=brson

This commit attempts to clarify a section of the tutorial. It also fixes some typos.
auto merge of #12183 : SimonSapin/rust/patch-4, r=alexcrichton
std -- replaces uses where const borrows would be required
remove antiquated reflect test rather than bring it up to date
libglob -- patch closure where const borrow would have helped
container -- update example to contain scope of closure borrow
to_str -- update to contain scope of closure
ty -- minor refactorings, helper methods
trans/datum -- move mutable variable into closure
error_reporting -- explain reborrowed upvar constraints in a hopefully useful way
metadata -- remove tiny convenience closure that was causing conflicting mutable borrows
resolve -- rewrite conflict closure into method
syntax/ext/format -- rewrite conflicting closures into methods