std: Remove i18n/l10n from format!

* The select/plural methods from format strings are removed
* The # character no longer needs to be escaped
* The \-based escapes have been removed
* '{{' is now an escape for '{'
* '}}' is now an escape for '}'

Closes #14810
[breaking-change]
It might have been nice to write these changes in a manner compatible with both the stage0 and stageN compilers.  (E.g. `try!(write!(w, "{}", "#"));`); since we did not do that, `make check-stage1` is currently broken (and presumably will remain so until the next snapshot...)