Potential improvements to Wasm brand types

[tlively]

Hi! I was taking a look at the brand type generation code and I noticed some opportunities for improvement.

1. The _brandTypeFieldValues could also include nullable and non-nullable references to i31, struct, and array, as well as i8 and i16.
2. The comments discuss working around structural subtyping issues, but WebAssembly does not have structural subtyping on defined types, so:
   1. It should be fine for a brand type to have zero fields.
   2. It should be fine to use mutable fields in the brand types, doubling the effective digits.

cc @natebiggs

[biggs0125]

Thanks for taking a look at this! This seems like low hanging fruit so I'll take a look at making these improvements.