Allow using Box<[u8]>/Box<str> for bytes/string fields

[poliorcetics]

This reduces the size of objects by one byte each time they're used, which can help quite a lot in some cases.

This is fully optional and the defaults have not changed.

[poliorcetics]

I saw #429 but it's not what I wanted since it asks for two levels of indirection, Arc<Vec<u8>> for example.

I can probably easily add Arc<str> and Arc<[u8]> if necessary though

[poliorcetics]

I tried to implement this in an external lib but it's inefficient if done naively (conversions to String/Vec<u8>) everywhere and since prost doesn't expose its helpers to build Messages, doing it efficiently means copying a lot of code that is intended for internal use only and is quite brittle when used in another crate.

[michaelvanstraten]

Please merge this, would be extremely helpful.

[poliorcetics]

Rebased to fix conflicts, no actual changes made