Finalize support for Nim 0.20

Closes #32, closes #41.

godot/internal/backwardcompat.inc.nim

@@ -0,0 +1,19 @@
+# Copyright 2019 Xored Software, Inc.
+
+when (NimMajor, NimMinor, NimPatch) < (0, 19, 0):
+  import macros, sets
+
+  proc strVal(n: NimNode): string {.used.} =
+    case n.kind
+    of nnkIdent:
+      $n.ident
+    of nnkSym:
+      $n.symbol
+    else:
+      macros.strVal(n)
+
+  proc toHashSet[A](keys: openArray[A]): HashSet[A] {.inline, used.} =
+    toSet(keys)
+
+  proc initHashSet[A](): HashSet[A] {.inline, used.} =
+    initSet[A]()

godot/internal/godotstrings.nim

@@ -38,7 +38,13 @@ proc `$`*(self: GodotString): string =
 
 proc toGodotString*(s: string): GodotString {.inline.} =
   ## Converts the Nim string into ``GodotString``
-  initGodotString(result, cstring(s), cint(s.len + 1))
+  when (NimMajor, NimMinor, NimPatch) < (0, 19, 0):
+    if s.isNil:
+      initGodotString(result)
+    else:
+      initGodotString(result, cstring(s), cint(s.len + 1))
+  else:
+    initGodotString(result, cstring(s), cint(s.len + 1))
 
 proc toGodotString*(s: cstring): GodotString {.inline.} =
   ## Converts the cstring into ``GodotString``