Support for generic types

[randomPoison]

Currently, it's not really possible to implement TypeUuid for a generic type. You can implement it for a concrete instantiation of a type, but not generically for all instantiations, i.e. you can implement it for Vec<String> and Vec<usize>, but there's no way to implement it generically for all Vec<T>. The way to do this would be to automatically generate a UUID for the concrete instantiation of the type based on the UUIDs of the input types. In theory this can be done with const functions by writing a function that takes a pair of Bytes and spits out a new Bytes that mixes the two together in a collision-resistant way.

[randomPoison]

I think the biggest blocker here is determining a reliable way to hash the input UUIDs in a const context. I think it should be possible to do so, however the existing hash crates don't seem to support const hashes (at least as far as I have seen), so we may have to make some upstream contributions or implement something ourselves.

[randomPoison]

After doing a bit more research, it looks like the current support for const function evaluation is so limited as to make it effectively impossible to implement this: https://github.com/rust-lang-nursery/reference/blob/3a7b6d863183079d46d92313df7875e22801de8d/src/items/functions.md#const-functions

We'll either need an alternate solution, or to wait until const function support is more robust.

[randomPoison]

So as of Rust 1.46.0 enough of the core language features are enabled for a const context that a const hash algorithm can be implemented. In fact, someone has already implemented sha1, which should be sufficient for our purposes!