This is an alternative to #32581, that is easier to implement, but
more restrictivive. Here, the forward declaration must contain
everything except for the field types:
```
incomplete type Foo{Bar} <: Baz; end
```
with it being an error to deviate from this specification when
completing the type
```
struct Foo <: Baz; end # Error: Missing type parameter
struct Foo{A, B} <: Baz; end # Error: Too many type parameters
struct Foo{Qux} <: Baz; end # Error: Name of type parameter doesn't match
struct Foo{Bar<:Int64} <: Baz; end # Error: Bounds of type parameter don't match
struct Foo{Bar}; end; # Error supertype dosesn't match
```
This is easier because this way we have enough information for subtyping
and type application and therefor do not need to delay this until
the entire dependency graph is complete as we did in #32581.
Of course this also means that we don't get the union feature that
was requested in #269:
```
inomplete type U; end
struct A; x::U; end
struct B; x::U; end
U = Union{A, B}; #ERROR
```
However, it could of course be emulated by wrapping the union type:
```
struct U
   data::Union{A, B}
end
```

However, given the simplicity of this change and the difficulty of #32581,
this seems like the way to go for the moment. We may want to revisit all
this if we ever want to do computed field types, which will encounter similar
challenges as #32581.

Fixes #269.