Support explicit type on bitdata

[jtdaugherty]

bitdata declarations with alternatives currently need to annotate at least one alternative with a type to indicate the width of all of the possible alternatives, e.g.,

bitdata Foo where
  X = 0x01: uint 8
  Y = 0x02: uint 8

This is repetitive and leads to redundant type annotations that Daedalus checks anyway, so if one alternative has a different width, you get an error like

  Mismatched constructor widths
    • constructor X has width 2
    • constructor Y has width 8

This ticket proposes that we instead allow the bitdata head itself to be annotated with a type that is then assigned to all of the alternatives, which then no longer need to be annotated explicitly. @yav and I discussed two possible syntax options:

bitdata Foo as uint 8 where
  ...

bitdata Foo: uint 8 where
  ...