[Binary format] Encode continuation type index as an s33 (WebAssembly#89)

There is a small inconsistency between the reference implementation in WebAssembly#84 and the explainer. In the explainer we write that the type index `$ft` on `cont $ft` should be encoded as an `u32` in the binary format. However, the reference interpreter actually encodes it as `s33`, similarly to how concrete heap types are encoded.

Author: dhil

proposals/stack-switching/Explainer.md

@@ -842,7 +842,7 @@ We extend the binary format of composite types, heap types, and instructions.
 | Opcode | Type            | Parameters | Note |
 | ------ | --------------- | ---------- |------|
 | -0x20  | `func t1* t2*`  | `t1* : vec(valtype)` `t2* : vec(valtype)` | from Wasm 1.0 |
-| -0x23  | `cont $ft`      | `$ft : typeidx` | new |
+| -0x23  | `cont $ft`      | `$ft : s33` | new |
 
 #### Heap Types