std bump

Stream: mathlib4

[Thomas Murrills]

@Gabriel Ebner and @Eric Wieser , just letting you both know that I've made a std bump PR which takes into account !4#2878.

Initially I started bumping std because I didn't see a PR which bumped std to include Rat.cast, but didn't realize it had been bumped in the nightly bump. (Nor did I simply search for Rat.cast! Silly me.) Anyway, just letting you know it's there in case it was on your to-do lists and saves either of you a couple of minutes! :) !4#2946

[Eric Wieser]

Thanks!