remove structural equality check for objects and distinct types (#24448)

follows up #24425, fixes #18861, fixes #22445

Since #24425 generic object and distinct types now accurately link back
to their generic instantiations. To work around this issue previously,
type matching checked if generic objects/distinct types were
*structurally* equal, which caused false positives with types that
didn't use generic parameters in their structures. This structural check
is now removed, in cases where generic objects/distinct types require a
nontrivial equality check, the generic parameters of the `typeInst`
fields are checked for equality instead.

The check is copied from `tyGenericInst`, but the check in
`tyGenericInst` is not always sufficient as this type can be skipped or
unreachable in the case of `ref object`s.

(cherry picked from commit a2031ec6cf)

tests/objects/tgenericsubtype.nim

@@ -16,3 +16,37 @@ block: # no generic field
   proc foo(x: typedesc[Foo]) = discard
 
   foo(Bar)
+
+block: # issue #22445
+  type
+    MyType = ref object of RootObj
+    MyChild[T] = ref object of MyType
+    
+  var curr = MyChild[int]()
+  doAssert not(curr of MyChild[float])
+  doAssert curr of MyChild[int]
+  doAssert curr of MyType
+
+block: # issue #18861, original case
+  type
+    Connection = ref ConnectionObj
+    ConnectionObj = object of RootObj
+    
+    ConnectionRequest = ref ConnectionRequestObj
+    ConnectionRequestObj = object of RootObj
+      redis: Connection
+    
+    ConnectionStrBool = distinct bool
+    
+    ConnectionRequestT[T] = ref object of ConnectionRequest
+    
+    ConnectionSetRequest = ref object of ConnectionRequestT[ConnectionStrBool]
+      keepttl: bool
+
+  proc execute(req: ConnectionRequest): bool = discard
+  proc execute(req: ConnectionRequestT[bool]): bool = discard
+  proc execute(req: ConnectionRequestT[ConnectionStrBool]): bool = discard
+
+  proc testExecute() =
+    var connection: ConnectionSetRequest
+    let repl = connection.execute()