Converting from BasicSchema to TypeLevelSchema

ACSets.jl allows us to go from `TypeLevelSchema` to `BasicSchema`, and I thought at some point I needed to go the other way around and wrote this one-liner:

```julia
""" Convert a Schema to a Type-level schema """
TypeLevelBasicSchema(s::BasicSchema{Name}) where {Name} =
TypeLevelBasicSchema{Name, Tuple{s.obs...}, Tuple{s.homs...},
Tuple{s.attrtypes...}, Tuple{s.attrs...}, Tuple{s.eqs...}}

TypeLevelBasicSchema(s::Type{<:TypeLevelBasicSchema}) = s
```

But then I happened to not need to. Still, this might be a feature that someone wants?