gfx: to_string() -> into_string()
style: to_string() -> into_string()
script: to_string() -> into_string()
layout: to_string() -> into_string()
servo: to_string() -> into_string()
auto merge of #4485 : servo/servo/into_string, r=Ms2ger

`str::to_string()` goes through a `Formatter`, `str::into_string()` is a direct copy and is apparently 5× faster.

This is a rebase of the boring and bitrot-prone parts of #4366.