Use target field types in tuple conversions.

tests/tuples/tconver_tuple.nim

@@ -0,0 +1,19 @@
+# Bug 4479
+
+type
+  MyTuple = tuple
+    num: int
+    strings: seq[string]
+    ints: seq[int]
+
+var foo = MyTuple((
+  num: 7,
+  strings: @[],
+  ints: @[],
+))
+
+var bar = (
+  num: 7,
+  strings: @[],
+  ints: @[],
+).MyTuple