implement generic default values for object fields (#24384)

fixes #21941, fixes #23594

tests/objects/tgenericdefaultvalue.nim

@@ -0,0 +1,26 @@
+block: # issue #23594
+  type
+    Gen[T] = object
+      a: T = 1.0
+
+    Spec32 = Gen[float32]
+    Spec64 = Gen[float64]
+
+  var
+    a: Spec32
+    b: Spec64
+  doAssert sizeof(a) == 4
+  doAssert sizeof(b) == 8
+  doAssert a.a is float32
+  doAssert b.a is float64
+
+block: # issue #21941
+  func what[T](): T =
+    123
+
+  type MyObject[T] = object
+      f: T = what[T]()
+
+  var m: MyObject[float] = MyObject[float]()
+  doAssert m.f is float
+  doAssert m.f == 123.0