Give BoundBlock, StreamValue and RawDataView more helpful repr()s

It seems quite likely that the lack of nice representations for these in the shell / debugger is what stopped people from realising that they could index StreamValue as an array, and led them towards stream_data instead ("ooh, this looks like an array of data... I know how to deal with this")

wagtail/core/blocks/base.py

@@ -472,6 +472,9 @@ def __str__(self):
         """Render the value according to the block's native rendering"""
         return self.block.render(self.value)
 
+    def __repr__(self):
+        return "<block %s: %r>" % (self.block.name or type(self.block).__name__, self.value)
+
 
 class DeclarativeSubBlocksMetaclass(BaseBlock):
     """

wagtail/core/blocks/stream_block.py

@@ -457,6 +457,9 @@ def insert(self, i, item):
             self.stream_value._raw_data.insert(i, item)
             self.stream_value._bound_blocks.insert(i, None)
 
+        def __repr__(self):
+            return repr(list(self))
+
     def __init__(self, stream_block, stream_data, is_lazy=False, raw_text=None):
         """
         Construct a StreamValue linked to the given StreamBlock,
@@ -606,7 +609,7 @@ def __len__(self):
         return len(self._bound_blocks)
 
     def __repr__(self):
-        return repr(list(self))
+        return "<%s %r>" % (type(self).__name__, list(self))
 
     def render_as_block(self, context=None):
         return self.stream_block.render(self, context=context)