Refactor: Geometry Nodes: add BundleKey type

Previously, a raw `UString` was used as bundle key. However, that did not
capture all the constraints put on the key (some characters are forbidden). This
patch adds a smaller `BundleKey` wrapper around `UString` that ensures these
constraints during construction so that code using the type does not have to
check it again.

This might also become useful when we want `Bundle` to become a thinner wrapper
around `Map` instead of duplicating large parts of its interface.

Pull Request: https://projects.blender.org/blender/blender/pulls/159023

tests/python/rna_info_dump.py

@@ -113,7 +113,7 @@ def api_dump(use_properties=True, use_functions=True):
         sys.stderr.write("\n\nEOF\n")
     else:
         text = bpy.data.texts.new(name="api.py")
-        text.from_string(data)
+        text.from_str(data)
 
     print("END")