Add __repr__ to Pythom Group and HorizonalChoice to fix #101 (#102)

* Group is missing a repr().

* HorizontalChoice is missing a repr().

* Update railroad.py

---------

Co-authored-by: Tab Atkins Jr <jackalmage@gmail.com>

railroad.py

@@ -1099,6 +1099,10 @@ def format(self, x: float, y: float, width: float) -> HorizontalChoice:
                 )
         return self
 
+    def __repr__(self) -> str:
+        items = ", ".join(repr(item) for item in self.items)
+        return f"HorizontalChoice({items})"
+
 
 def Optional(item: Node, skip: bool = False) -> Choice:
     return Choice(0 if skip else 1, Skip(), item)
@@ -1219,6 +1223,9 @@ def walk(self, cb: WalkerF) -> None:
         if self.label:
             self.label.walk(cb)
 
+    def __repr__(self) -> str:
+        return f"Group({repr(self.item)}, label={repr(self.label)})"
+
 
 class Start(DiagramItem):
     def __init__(self, type: str = "simple", label: Opt[str] = None):