Adjust to upcoming base Julia change

This adjusts the `@system` macro to be compatible with
JuliaLang/julia#56497, which
re-orders the point at which a type is published into
the bindings table until after it is complete. This
package was (uniquely among all julia packages) relying
on the binding existing during field definition (via
a call to `typefor`).

Unfortunately, there isn't really a great place to define
as the system (currently) does not gurantee an order of
the evaluation of field types with respect to other global
statements inside the struct evaluation. The only place
that satiesfies the criteria that
a) incomplete struct is avilable
b) it is evaluated before any field types
is the supertype declaration, so this PR shoves the definition
of `typefor` there.

This works both before and after the change, although
it is quite messy. In the future, there may be a way
to forward declare an incomplete type, but for the time
being, this is the best I could come up with.

src/macro.jl

@@ -356,7 +356,7 @@ end
 
 genstate(v::VarInfo) = begin
     name = Meta.quot(v.name)
-    alias = Meta.quot(v.alias)    
+    alias = Meta.quot(v.alias)
     value = istag(v, :extern) ? genextern(v, gendefault(v)) : gendefault(v)
     stargs = [:($(esc(k))=$v) for (k, v) in filterconstructortags(v)]
     @q $C.$(v.state)(; _name=$name, _alias=$alias, _value=$value, $(stargs...))
@@ -459,9 +459,14 @@ genstruct(name, type, infos, consts, substs, incl, scope) = begin
     system = quote
         Core.@__doc__ abstract type $S <: $T end
         $S(; kw...) = $_S(; kw...)
-        $C.typefor(::Type{<:$S}) = $_S
         let $(constpatches...)
-            Core.@__doc__ mutable struct $_S <: $S
+            # HACK: We want to define `typefor` before the type is fully defined
+            # because it may be used in the `fields`. Because of the order that
+            # struct definitions happen to get evaluated in, type supertype definition
+            # is the only place where we have access to the incomplete $_S before evaluating
+            # the fields. The extra let is required to due a lowering bug
+            # (https://github.com/JuliaLang/julia/issues/56510).
+            Core.@__doc__ mutable struct $_S <: ((let $_S=$_S; ($C.typefor(::Type{<:$S}) = $_S); end); $S)
                 $(fields...)
                 function $_S(; __kwargs__...)
                     $predecl