[EH] Make tag attribute's encoding uint8 (#3949)
[EH] Make tag attribute's encoding uint8 (#3949)

This changes the encoding of the `attribute` field, which currently only
contains the value `0` denoting this tag is for an exception, from
`varuint32` to `uint8`. This field is effectively unused at the moment
and reserved for future use, and it is not likely to need `varuint32`
even in future.
See https://github.com/WebAssembly/exception-handling/pull/162.

This does not change any encoded binaries because `0` is encoded in the
same way both in `varuint32` and `uint8`.
WebAssembly/binaryen,WebAssembly/binaryen,WebAssembly/binaryen,WebAssembly/binaryen,WebAssembly/binaryen