We need to be able to pass more context to the model

[Yoric]

I'm trying to use redux to implement a compression mechanism for sophisticated data structures. For this purpose, I need to be able to pass potentially complex context to the model, not just usize.

For this purpose, I'd need something along the lines of:

/// Values of type `T` contains all the necessary information we need to write to
/// the bit stream. Values are eventually converted to a `usize` for actual writing.
struct Codec<T> where T: Hash + Eq + Into<usize> {
  // ...
}
impl<T> Codec<T> where T: Hash + Eq + Into<usize> {
  fn compress_symbol(&mut self, context: T, output: &mut BitWrite) -> Result<()>;
}

// Similar changes to `Model`.

Instead of the current implementation that hardcodes usize everywhere.

What do you think?