Add non-`unsafe` `.get_mut()` for `UnsafeCell`
Add non-`unsafe` `.get_mut()` for `UnsafeCell`

Update the tracking issue number

Updated the documentation for `UnsafeCell`

Address review comments

Address more review comments + minor changes

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