Modify CloudABI ReentrantMutex to use MaybeUninit
Modify CloudABI ReentrantMutex to use MaybeUninit

Remove uses of mem::uninitialized, which is now deprecated
graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust