Rollup merge of #37065 - nnethercote:opt-mk_printer, r=nikomatsakis

Merge `Printer::token` and `Printer::size`.

Logically, it's a vector of pairs, so might as well represent it that
way.

The commit also changes `scan_stack` so that it is initialized with the
default size, instead of the excessive `55 * linewidth` size, which it
usually doesn't get even close to reaching.