[Parser] Update requirements for implicit type uses

As an abbreviation, a `typeuse` can be given as just a list of parameters and
results, in which case it corresponds to the index of the first function type
with the same parameters and results. That function type must also be an MVP
function type, i.e. it cannot have a nontrivial rec group, be non-final, or have
a declared supertype. The parser did not previously implement all of these rules.

Author: tlively

src/parser/contexts.h

@@ -1119,7 +1119,7 @@ struct ParseImplicitTypeDefsCtx : TypeParserCtx<ParseImplicitTypeDefsCtx> {
       types(types), implicitTypes(implicitTypes) {
     for (auto type : types) {
       if (type.isSignature() && type.getRecGroup().size() == 1 &&
-          !type.isShared()) {
+          !type.getDeclaredSuperType() && !type.isOpen() && !type.isShared()) {
         sigTypes.insert({type.getSignature(), type});
       }
     }