bug#70794: 30.0.50; Add Rust compilation regex

[Mattias Engdegård]

There is also already a regexp that looks like it is used to match similar 
messages in `rust-ts-flymake`, but it lacks some of the problems listed above. 
Maybe they could be consolidated?

There is another problem with the added `cargo` regexp: its placement early in 
`compilation-error-regexp-alist-alist` causes it to match before other, 
possibly more frequently occurring patterns.
(That is not your (or Eli's) fault but an undocumented subtlety of that 
variable; I'll add a remark to the code.)

In fact the added pattern has visibly wrecked the display of 
etc/compilation.txt so I'm taking the slightly drastic step of reverting it. It 
seems we'll need to work harder on this one -- sorry.