rustfmt: manually split up string literals crossing the maximum line width.

[eddyb]

There's an unfortunate rustfmt bug/limitation, where string literals that cross the max width breaks reformatting in everything it's nested in.

The resulting files were tested for reformatting ability by (un)indenting the entire file (e.g. Ctrl+A Tab Ctrl+S in VSCode), and inspecting the resulting diff - complete success leads to only whitespace changes in lines that are string \ continuations (because rustfmt doesn't touch those).

In fact, to ensure all instances are caught, I ended up doing this instead:

sed -E -i 's/^./ \0/' src/**.rs && cargo fmt --all && git diff

(sadly, macros also don't get reformatted, so this is a bit annoying to skim, but still useful)

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You can see in the diff that there was some positive overall impact by looking for formatting changes that are not directly around the changed strings, but rather in nearby (sibling-ish) AST nodes.