Rollup merge of #47440 - mark-i-m:zunpretty, r=nikomatsakis

Rollup merge of #47440 - mark-i-m:zunpretty, r=nikomatsakis

Change the --unpretty flag to -Z unpretty

First PR :smile: !

-Z unpretty no longer requires -Z unstable-options.

Also, I mildly changed the syntax of the flag to match the other -Z flags. All uses of the flag take the form `unpretty=something` where something can either `string` or `string=string` (see the help messages of the CLI).

Fix #47395

r? @nikomatsakis EDIT: apparently rust-highfive doesn't see edits...