Fix parser of declarative element segments

The parser was incorrectly handling the parsing of declarative element segments (those with the `elemkind` of `declarative`).
According to the spec, the `init` of the declarative element should be a `vec(expr)` if the prefix is 7.
https://webassembly.github.io/spec/core/binary/modules.html#element-section
However, binaryen was simply reading a single `u32LEB` value for each
expression instead of parsing a expression regardless `usesExpressions = true`.

This commit updates the `WasmBinaryReader::readElementSegments` function
to correctly parse the expressions for declarative element segments by
calling `readExpression` instead of `getU32LEB` if `usesExpressions = true`.

Resolves the parsing exception:
"[parse exception: bad section size, started at ... not being equal to new position ...]"

Related discussion: tanishiking/scala-wasm#136

src/wasm/wasm-binary.cpp

@@ -3354,7 +3354,11 @@ void WasmBinaryReader::readElementSegments() {
       [[maybe_unused]] auto type = getU32LEB();
       auto num = getU32LEB();
       for (Index i = 0; i < num; i++) {
-        getU32LEB();
+        if (usesExpressions) {
+          readExpression();
+        } else {
+          getU32LEB();
+        }
       }
       continue;
     }