Performance: low hanging fruit

[epage]

Whenever we parse, we have to filters.clone(). This could be sped up by putting Parser::filters into an Arc.

Then whenever we render, we can pass the Arc in rather than doing another filters.clone().

Other efforts

• Using Cow in Value/Object, see Support 'static str/Cow<str> for Object::insert(...) #186
• Using a Writer when rendering, see Render to Writer #187
• Don't clone globals, see RFC: API Refactor #95