Add a standard library stabilization "things to double-check" list

[scottmcm]

The "if it's a new trait it needs to have &mut blankets immediately" problem came up on IRLO recently: https://internals.rust-lang.org/t/implement-std-asrawfd-for-references/20132/5?u=scottmcm

We've been better about this for newer traits, but it reminded me that it would be good to have a list of "stuff we can't fix later" to double-check for every FCP, which would also be a good resource for people making ACPs to think about.