Reserve `do` as a keyword

Resolves issue #12157. `do` is hereby reinstated as a keyword; no syntax is
associated with it though. Along the way, a unit test had to be adapted, since
it was using `do` as a method identifier.

Breaking changes:

- Any code using `do` as an identifier will no longer work.
auto merge of #12170 : aepsil0n/rust/feature/reserve_do_keyword, r=brson

This resolves issue #12157. Does that do it already or is there something else that needs taking care of?  

As a side note, there seems to be some documentation, in which the old existence of the do keyword is explained. The list of keywords is not up-to-date either. But these are certainly separate issues.