Support unicode aliases in `@present`

We should probably support the unicode aliases in the declaration of a presentation.

```julia
@present Systems(FreeSymmetricMonoidalCategory) begin
A::Ob
f::(A\to A)
end
```

If we combine this with a general approach to allowing GATs with metatypes like `Element` or `Variable` we could really make the Catlab user code look like regular math!