Indicate to the compiler that Rooted<T> may be uninitialized

[simonwuelker]

Wraps Rooted::ptr in MaybeUninit so it can safely be zero-initialized.

Fixes #548

[simonwuelker]

These changes also remove the Initialize bound on RootedGuard, because it is no longer needed (instead of being initialized to a default value, the ptr field is now uninitialized).

I think this is an improvement, because it is not always possible to cheaply construct default values for Initialized, for example if the type contains Box or similar things that need to be allocated.

[jdm]

This is subtle but looks like a solid fix for the issue that you identified. My only concern is with the set method.