Replace `SyntaxKind` with `Into<SyntaxKind>` in function parameters

[spencerwhite]

It would be much more convenient to write builder.start_node(Ast::Expr) than builder.start_node(Ast::Expr.into()), assuming there is an impl From<Ast> for SyntaxKind somewhere. Obviously not a huge deal, but it would slightly cut down on boilerplate.