Add Mutex::into_inner method

[WaffleLapkin]

Add Mutex::into_inner method that consumes mutex
and return underlying value.

I'm not sure what to test here, so I didn't add any tests.

Fixes #2211

Motivation

There are cases when you need to get ownership over the underlying value.

Also, both std::sync::Mutex and futures::sync::Mutex have the methods into_inner that allow you to consume the mutex and get the owned value from it.

Solution

Add into_inner method for sync::Mutex that simply calls into_inner on inner UnsafeCell

[LucioFranco]

This seems fine 👍 thanks!