Cleanup to syntax::print/rustc::hir::print

[Mark-Simulacrum]

This is in preparation for a removal of io::Result<()> from all of the printing infrastructure methods (separate PR due to huge diff), along with potentially some other cleanups.

r? @Centril (but feel free to reassign if there's someone better -- there's no functional changes here)

[rust-highfive]

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[Centril]

I don't understand the larger picture of excising io::Result<()> from pretty printing.
However, assuming you do and the &mut Vec<u8> are going towards a goal you understand...
...the changes in the PR look fine.

So r=me on that.

[Centril]

That said, cc @petrochenkov and @eddyb.

[Mark-Simulacrum]

The larger goal is mostly just a simplification of the code, which may lead to both compile time wins (? on every other line of code isn't cheap due to expansion and later processing, I believe) and potentially performance wins. We can hold off on landing this PR until we get numbers, but I think it's pretty much an improvement without any significant downsides.

[petrochenkov]

@bors r+

[bors]

📌 Commit 0091e6e has been approved by petrochenkov

[eddyb]

Why not a String?