Idiomatic: Fix various broken code examples

[randomPoison]

Many of the code snippets in Idiomatic are broken in various ways that make them either not run in the slides or not show the error the slide is intended to illustrate. This PR fixes a number of common issues:

• Invalid syntax, e.g. let inside a struct definition.
• Missing main fn. Not all examples need a main, but several code examples had code that only compiles if it's in a function, and so was failing to compile in the slides. Note that these were not caught in tests because there seems to be a difference in behavior between how we test the code vs how the code works in the slides.
• Missing fn keyword on method definitions.

In a few places I made more opinionated changes that I'm including here because splitting them out into a separate PR would just result in merge conflicts later:

• hash.md - Removed the friends field. It wasn't necessary for demonstrating the Hash trait, and removing it allows the slide to be more concise.
• newtype_pattern.md - Tweak the exercise to show that you can't pass an inner type where a newtype wrapper is expected (inverse of what it was previously demonstrating). I think this is a slightly clearer way to show what the slide is demonstrating.
• A few minor tweaks to formatting.
• Remove a few extraneous comments.
• Add empty lines to help space things out and make them easier to read.
• Remove pub keyword in a few places.
• Remove the # prefix on lines that I think should be kept visible in the code snippet (though note that none of the hidden lines are ever hidden because the code snippets are editable, see Examples with hidden lines should be editable #2811).

[mgeisler]

Thanks for fixing the formatting (and all the other things!)

It just occurred to me that the # syntax very likely is what prevents dprint from formatting this code block. It knows it's Rust from the info string and it knows to split on ,, but rustfmt will not be happy with the # lines.

Cc @gribozavr as another side-effect of #3101.

[mgeisler]

Thank you for bringing this up to standard.