Try to optimize Key::encode

[uint]

It might be worthwhile to return something like Cow from Key::encode. Some common cases (String?) don't need us to compute a value.

[webmaster128]

I think the most prominent use case for such an optimization the use of Addr in keys (map from user to whatever). Using Addr::as_bytes (returns &[u8]) when composing the final key save an intermediate allocation of 40-65 bytes for the address.

[webmaster128]

There can also be a 3rd option here, data in arrays:

enum Key<'a> {
    Borrowed(&'a [u8]),
    Short1([u8; 1]), // can store 8 bit value
    Short2([u8; 2]), // can store 16 bit value
    Short4([u8; 4]), // can store 32 bit value
    Short8([u8; 8]), // can store 64 bit value
    Owned(Vec<u8>)
}

This enum should be 12 bytes large in Wasm (same as Vec<u8>) due to optimizations. I tested this in the playground but only on a 64bit target.