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".
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.
std: Fix search-and-replace typos
rustpkg: Extract version number from git, as per #5684

For now, the test I added just checks that PkgId::new parses the
version number out of a git repo's tags list, where relevant.
rustpkg: Accept package IDs like github.com/foo/bar#0.3

If the package ID is of the form s#v, where v is a valid version
string, fetch tag v of that package.