Remove slice pattern from compiler plugin example

[fhahn]

PR for #29930. Adds missing #![feature(slice_patterns)] to make compiler plugin example compile again.

[rust-highfive]

r? @nikomatsakis

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[arielb1]

I would prefer to remove the slice pattern - slice patterns are dangerous.

[fhahn]

Sure, I've updated the commit though I am not sure if there is a nicer way to replace the slice pattern.

I also noticed that src/test/auxiliary/roman_numerals.rs and the example in the book used different integer types (one used usize and one u32), my commit also updates the example to use usize as well. I hope that's OK. If not I would be happy to update the PR again.

[steveklabnik]

@bors: r+ rollup

[bors]

📌 Commit c53722d has been approved by steveklabnik

[steveklabnik]

Thanks!

[bors]

⌛ Testing commit c53722d with merge 6827661...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt