replace all remaining `for` with `foreach` or `do`
make `for` parse as `foreach` does

Closes #6997
rm obsolete documentation on `for`

it is documented in the container/iterator tutorial, not the basic
tutorial
Merge pull request #8244 from thestinger/for

make `for` parse as `foreach` does

r=huonw, bors is acting up and this has been run through the try bots