Avoid copying content when writing files

[dzbarsky]

A well-behaved FS implementation should not mutate the bytes to be written. (Also, it seems like many call sites already have an []byte and convert to a string just to call WriteFile)

[jakebailey]

(Also, it seems like many call sites already have an []byte and convert to a string just to call WriteFile)

For fun, I went to try and refactor file writing to see if using []byte were possible all the way, but it is a lot of changes. Reading as strings is pretty intentional so things kinda bubble from there. But maybe it's possible.

It'd be interesting to see your benchmarks for your PRs.