Make `from_waker`, `waker` and `from_raw` unstably `const` Make - `Context::from_waker` - `Context::waker` - `Waker::from_raw` `const`. Also added a small test.
Make `from_waker`, `waker` and `from_raw` unstably `const` Make - `Context::from_waker` - `Context::waker` - `Waker::from_raw` `const`. Also added a small test.