///|
/// Minify an HTML string.
///
/// - Keeps `` comments verbatim (untouched, whitespace and all).
/// - Collapses any run of whitespace into a single space.
/// - Drops whitespace entirely when it sits between two tags (e.g.
///   `   
` becomes `
`), while `>  hi` keeps the single
///   space before "hi".
/// - Copies the contents of `