Mention `for` in the section on loops

The "4.3 Loops" section only describes `while` and `loop`. We then see `for`
used in a code sample at the end of the "13. Vectors and strings" section,
but it's explained for the first time only in the next section --
"14. Closures".

It is worth mentioning it in "4.3 Loops".
std: Change str::from_bytes to raise a condition on invalid input

As per #4765
auto merge of #7001 : alco/rust/tutorial-for-loop, r=bstrie

The "4.3 Loops" section only describes `while` and `loop`. We then see `for`
used in a code sample at the end of the "13. Vectors and strings" section,
but it's explained for the first time only in the next section --
"14. Closures".

It is worth mentioning it in "4.3 Loops".

---

Feel free to reword it and commit yourself if you don't like my wording.
testsuite: Add test cases that pass
auto merge of #6695 : catamorphism/rust/moretestcases, r=catamorphism
auto merge of #6920 : catamorphism/rust/issue-4765, r=bstrie

r? anybody As per #4765
rustc::middle::trans::reachable: remove some unneeded code

This was a remnant from the export list era.
tests: make lots of main functions public