Remove DerefMut impl for Mutable Handle

[gmorenz]

Removes the deref_mut impl, and replaces it with an unsafe as_mut method. As discussed in #569 (and other recent issues) deref_mut is unsafe since it aliases with the GC, and cannot be made safe or marked unsafe.

The accompanying servo change (servo/servo#36160) does not need the as_mut method, it feels like an appropriate escape hatch to include though.

It also includes a MutableHandle::<Option<_>>::take method, since like in #572 that's a safe method, and #572's servo change results in it being used on MutableHandle.

Independent from (the obviously related) #572 and the similar incoming PR for JS::MutableHandle. Doing these all at roughly the same time feels like the right move, since otherwise we might accidentally encourage someone to convert from one of these types another to access the DerefMut function instead of using actual alternatives.

[gmorenz]

Oops, this is not actually independent from #572. #572 creates an instance where MutableHandle::<Option<_>>::take is called... I'll fix that like I fixed the many more take calls in #572 by adding a take helper function. (Updated so it is independent again)