Mutating functions are not supported on structs

Marking a mutating function on a structure @JS results in a compilation failure in the generated code.

For example, this struct generates code which fails to compile.

@JS struct Counter {

    var number: Int

    @JS public mutating func increment() {
        number += 1
    }

}

@_expose(wasm, "bjs_Counter_increment")
@_cdecl("bjs_Counter_increment")
public func _bjs_Counter_increment() -> Void {
    #if arch(wasm32)
    Counter.bridgeJSLiftParameter().increment() // error: cannot use mutating member on immutable value: function call returns immutable value
    #else
    fatalError("Only available on WebAssembly")
    #endif
}

先从 issue 中所示的 @JS struct 处理和生成的 _bjs_Counter_increment wrapper 开始，然后使用 Counter 示例重现编译失败。完成标准是：带有 mutating function 的 @JS struct 能生成可为 WebAssembly 成功编译的代码。