No longer put cloudabi mutexes in a box.
No longer put cloudabi mutexes in a box.

Cloudabi mutexes may be moved safely.

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