Implement a derive macro for storable structs

[uint]

The idea here is something like this:

#[derive(Storable)]
struct Foo {
    #[storey(key = "0")]
    bar: Item<u128>,
    #[storey(key = "1")]
    baz: Map<str, Item<u128>>,
}

This should generate a Storable implementation for Foo, with every field being under a different one-byte key.

One use of this is to give Column the ability to store multiple "cells" per row, implementing a sort of "table".