mutable function adaptor allows using a non-const function object
inside of a const-function object. In Fit, all the function adaptors use
const call overloads, so if there is a function that has a non-const
call operator, it couldn’t be used directly. So,
mutable_ allows the
function to be used inside of the call operator.
NOTE: This function should be used with caution since many functions are copied, so relying on some internal shared state can be error-prone.
template<class F> mutable_adaptor<F> mutable_(F f)