Rollup merge of #149869 - joboet:torn-dbg, r=Mark-Simulacrum

std: avoid tearing `dbg!` prints

Fixes rust-lang/rust#136703.

This is an alternative to rust-lang/rust#149859. Instead of formatting everything into a string, this PR makes multi-expression `dbg!` expand into multiple nested matches, with the final match containing a single `eprint!`. By using macro recursion and relying on hygiene, this allows naming every bound value in that `eprint!`.

CC @orlp

r? libs

Author: matthiaskrgr