Don't ignore warnings in html5ever/src/tree_builder

[simonwuelker]

Currently, html5ever/src/tree_builder/mod.rs contains this:

html5ever/html5ever/src/tree_builder/mod.rs

Line 10 in e6f0018

#![allow(warnings)]

Presumably, this is done to ignore all the warnings caused by the rules.rs file generated at build time (Which is simply included in mod.rs).

However, this masks many warnings from the actual code itself. This PR instead puts the generated code into its own module, ignores warnings only for this specific module and then fixes (almost) all the warnings in the rest of the code.

#[allow(warnings)]
mod autogenerated {
    include!(concat!(env!("OUT_DIR"), "/rules.rs"));
}

There are two warnings that are not fixed by this PR:

• all the Token variants ending with Token themselves (Token::TagToken, Token::EOFToken...). I didn't fix this because these variants are used directly without the enum name (TagToken, EOFToken...). Not a fan, but outside of the scope of this PR
• Some enum variants are never constructed, I've ignored this in simonwuelker@86dcdeb since I'm not sure if these can actually be removed

I've made individual commits for each "kind" of lint being addressed - I can squash these together if preferred.

[mrobinson]

Nice!