This does introduce some not-so-nice things, like having to annotate each
`__init__` function with `-> None`. However, the benefits of having static
type checking in a complex bit of software like BAT outweigh the downsides.
Type refinement is now only done with BlendFileBlock.refine_type(), and
no longer with sdna_index_refine parameters to various functions. This
simplifies the API at the expense of having to call two simple functions
instead of one more complex one.
