Catherine: i hacked up a patch last night to allow the use of Annotated[] in data.Flow members, for those users who wish to have full type checking available for their codebase. Would it be of interest if I finish that up properly, and would that require an RFC? Thanks!

e.g.
```
  class Command(data.Struct):
      class Kind(enum.Enum):
          SET_ADDR = 0
          SEND_DATA = 1

      valid: Annotated[Value, unsigned(1)]
      kind: Kind
      #
      params: Annotated[Value, data.UnionLayout({
          "set_addr": data.StructLayout({
              "addr": unsigned(32)
          }),
          "send_data": data.StructLayout({
              "byte": unsigned(8)
          })
      })]
```