Remove `unsafe` qualifier from `RefCell::as_unsafe_cell`
Remove `unsafe` qualifier from `RefCell::as_unsafe_cell`

This method is no longer unsafe because the field of `UnsafeCell` is no
longer public.

graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust