implements 'export' feature

tests/compile/mexporta.nim

@@ -0,0 +1,8 @@
+# module A
+import mexportb
+export mexportb.TMyObject, mexportb.xyz
+
+export mexportb.q
+
+proc `$`*(x: TMyObject): string = "my object"
+

tests/compile/mexportb.nim

@@ -0,0 +1,7 @@
+# module B
+type TMyObject* = object
+
+const xyz* = 13
+
+proc q*(x: int): int = 6
+proc q*(x: string): string = "8"

tests/compile/texport.nim

@@ -0,0 +1,10 @@
+discard """
+  output: "my object68"
+"""
+
+import mexporta
+
+# B.TMyObject has been imported implicitly here: 
+var x: TMyObject
+echo($x, q(0), q"0")
+