Block type indices must be encoded as a sleb

[eqrion]

[1] If not, when the number of types in a module grows too large, a uleb encoding will begin to be confused with a value type [3]. I believe the issue is here, where a type index is always encoded as a uleb [2].

[1] https://webassembly.github.io/multi-value/core/binary/instructions.html#control-instructions
[2]

wat/crates/wast/src/binary.rs

Line 257 in e94a399

Index::Num(n) => n.encode(e),

[3] https://searchfox.org/mozilla-central/rev/49ed791eec93335abfe6c2880f84c324e73e47e6/js/src/jit-test/tests/wasm/multi-value/block-validate.js#174

[alexcrichton]

Oh man y'all got good tests :)