Format strings

[eduardosm]

Otherwise, when there were long strings, rustfmt was skipping the whole block.

Used:

cargo fmt -- --config format_strings=true

[psychon]

Do we need to drop a file somewhere with configuration for rustfmt to always enable this?

[eduardosm]

Options can be set in a file named rustfmt.toml. Note that, like the format_code_in_doc_comments, the format_strings options is "unstable" (rust-lang/rustfmt#3353). I believe that this means that the behavior might change. So, for now I'd just do a one-time run when we spot something wrong.