extract forks definition (all but rename v2forks.nim)

why:
  new name forks_list.nim file name matches additional documentation
  file names.

details:
  v2forks.nim remains a hollowed out shell serving as interface file.

nimbus/vm2/interpreter/forks_list.nim

@@ -0,0 +1,30 @@
+# Nimbus
+# Copyright (c) 2018 Status Research & Development GmbH
+# Licensed under either of
+#  * Apache License, version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or
+#    http://www.apache.org/licenses/LICENSE-2.0)
+#  * MIT license ([LICENSE-MIT](LICENSE-MIT) or
+#    http://opensource.org/licenses/MIT)
+# at your option. This file may not be copied, modified, or distributed except
+# according to those terms.
+
+## List of known Etheroum forks
+## ============================
+##
+## See `here <../../ex/vm/interpreter/forks_list.html>`_ for an
+## overview.
+##
+
+type
+  Fork* = enum
+    FkFrontier = "frontier"
+    FkHomestead = "homestead"
+    FkTangerine = "tangerine whistle"
+    FkSpurious = "spurious dragon"
+    FkByzantium = "byzantium"
+    FkConstantinople = "constantinople"
+    FkPetersburg = "petersburg"
+    FkIstanbul = "istanbul"
+    FkBerlin = "berlin"
+
+# End

nimbus/vm2/interpreter/v2forks.nim

@@ -10,16 +10,7 @@ when not defined(vm2_enabled):
 when defined(evmc_enabled):
   {.fatal: "Flags \"evmc_enabled\" and \"vm2_enabled\" are mutually exclusive"}
 
-import stint, eth/common/eth_types
+import forks_list
+export Fork
 
-type
+# End
-  Fork* = enum
-    FkFrontier = "frontier"
-    FkHomestead = "homestead"
-    FkTangerine = "tangerine whistle"
-    FkSpurious = "spurious dragon"
-    FkByzantium = "byzantium"
-    FkConstantinople = "constantinople"
-    FkPetersburg = "petersburg"
-    FkIstanbul = "istanbul"
-    FkBerlin = "berlin"