Macro digest::impl_opaque_debug

macro_rules! impl_opaque_debug {
    ($state:ty) => { ... };
}

Macro for defining opaque Debug implementation. It will use the following format: "HasherName { ... }". While it's convinient to have it (e.g. for including in other structs), it could be undesirable to leak internall state, which can happen for example through uncareful logging.