Create initial subfield macro and tests

tests/test_subfield.nim

@@ -0,0 +1,63 @@
+import einheit
+import subfield
+
+testSuite SubFieldTests:
+  method testOneLevel() =
+
+    type
+      C = object
+        x: int
+      B = object
+        c: C
+      A = object
+        b: B
+
+    var c = C(x: 20)
+    var b = B(c: c)
+    var a = A(b: b)
+
+    self.check compiles(a.x)
+    self.check a.x == 20
+
+  method testTwoLevels() =
+    type
+      D = object
+        y: int
+      C = object
+        d: D
+      B = object
+        c: C
+      A = object
+        b: B
+
+    var d = D(y: 200)
+    var c = C(d: d)
+    var b = B(c: c)
+    var a = A(b: b)
+
+    self.check compiles(a.y)
+    self.check a.y == 200
+
+  method testBranch() =
+    type
+      D = object
+        y: int
+      C = object
+        x: int
+      B = object
+        c: C
+        d: D
+      A = object
+        b: B
+
+    var d = D(y: 200)
+    var c = C(x: 19)
+    var b = B(c: c, d: d)
+    var a = A(b: b)
+
+    self.check compiles(a.y)
+    self.check a.y == 200
+
+    self.check compiles(a.x)
+    self.check a.x == 19
+