Reuse boxed values in `add_value`

[Thomasdezeeuw]

Todo in the code:

io-context/src/lib.rs

Line 378 in 7740d1e

// TODO: what if `V` is already boxed, can be reuse to box somehow?

Easier way to do it (I think) is to create a new method:

pub fn add_boxed_value<V>(&mut self, key: &'static str, value: Box<V>)