Make `Symbol` derive from internal `.types.Struct`

piker/data/_sampling.py

@@ -331,7 +331,6 @@ async def sample_and_broadcast(
             lags: int = 0
 
             for (stream, ctx, tick_throttle) in subs:
-
                 try:
                     with trio.move_on_after(0.2) as cs:
                         if tick_throttle:

piker/data/_source.py

@@ -23,7 +23,8 @@ import decimal
 
 from bidict import bidict
 import numpy as np
-from msgspec import Struct
+
+from .types import Struct
 # from numba import from_dtype