Use internal mutability for Event.
Use internal mutability for CustomEvent.
Use internal mutability for UIEvent.
Use internal mutability for EventTarget.
Use internal mutability for HTMLIFrameElement.
Use internal mutability for Node.flags.
Stop mutating Element::namespace.
Use internal mutability for CharacterData::data.
Use internal mutability for Window::browser_context.
Use internal mutability for Window::{active_timers, next_timer_handle}.
Use internal mutability for Document::{quirks_mode, encoding_name}.
Use internal mutability for FormData.
Use internal mutability for HTMLImageElement.
auto merge of #2633 : Ms2ger/servo/derefmut, r=pcwalton

Part of #1854.