Engine: Omit `__herb = ::Herb::Engine;` in compiled template if it's not needed

[marcoroth]

Compiling a HTML+ERB document using the Herb::Engine always emits the __herb = ::Herb::Engine; preamble.

For example in a document like:

<h1>
  <% if true %>
    <div>Text1</div>
  <% elsif false %>
    <div>Text2</div>
  <% else %>
    <div>Text3</div>
  <% end %>
</h1>

This gets compiled to:

__herb = ::Herb::Engine; _buf = ::String.new;
 _buf << '<h1>
  '.freeze; if true; _buf << '
    <div>Text1</div>
  '.freeze; elsif false; _buf << '
    <div>Text2</div>
  '.freeze; else; _buf << '
    <div>Text3</div>
  '.freeze; end; _buf << '
</h1>
'.freeze;
_buf.to_s

We could omit the preamble if we notice that we aren't using it in the compiled Ruby template.