The `uv_*_t` handle types are closed by `uv_close`, but the semantics
are tricky. Calling `uv_close` may not close immediately. Instead it
hands ownership to the uv loop to which the handle is currently
attached. When the loop decides to close it, a callback is used to
allow the `uv_close` caller to free resources.
Provide an abstraction layer as `cm::uv_*_ptr` types corresponding to
the `uv_*_t` handle types. Each pointer is either empty (`nullptr`)
or has an initialized handle attached to a loop. Use move semantics
to ensure a single owner of the handle so that clients can predict
when the handle is destroyed.
