Auto merge of #59507 - nnethercote:indent-with-SPACES, r=petrochenkov
Auto merge of #59507 - nnethercote:indent-with-SPACES, r=petrochenkov

Optimize indentation in the pretty printer.

Currently the pretty-printer calls `write!` for every space of
indentation. On some workloads the indentation level can exceed 100, and
a faster implementation reduces instruction counts by up to 7% on a few
workloads.

graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust