Macro transfer

macro_rules! transfer {
    (|| $block:block) => { ... };
    (|$($param:ident: $ty:ty),*| $block:block) => { ... };
}

On WASM, JsValues cannot be shared between scopes but instead can be “transferred”. Rust however is not aware of transferables and therefore cannot capture these values. This macro wraps a closure and returns a TransferClosure on WASM platforms which will capture these special values, or a normal FnOnce on other platforms. Note that the parameter names must match available variables/bindings from the outer scope.