Add unrooted_must_root lint for enums and structs containing JS<T>, as well as functions with JS<T> in their parameter list

For safe wrappers over JS<T> (eg Temporary<T>) use #[allow(unrooted_must_root)].
For all other types containing a #[must_root] value, annotate the type with #[must_root] to ensure that it is never used unrooted
Add unrooted_must_root lint for usages of JS<T> in let/for bindings
Unwrap pointers and miscellany for unrooted_must_root check
Address review comments
Make Reflector #[must_root], propagate to non-HTMLElements
Use #[must_root] for HTMLElements
Merge pull request #3374 from Manishearth/lint_unrooted_jsmanaged

Add lint for ensuring proper rooting of JS<T>; r=jdm