Update ArcWaker::into_waker() call

[jakmeier]

Syntax of function has been changed
See rust-lang/futures-rs#1676

[jakmeier]

Hi, I suggest this PR to fix the executors for futures to work with the latest nightly.
Currently, stdweb with futures-support does not compile.

To reproduce:
cargo web check --features=futures-support
Results in:

error[E0599]: no function or associated item named `into_waker` found for type `dyn futures_util::task::ArcWake` in the current scope
  --> src/webcore/executor.rs:74:34
   |
74 |             let waker = ArcWake::into_waker( arc.clone() );
   |                                  ^^^^^^^^^^ function or associated item not found in `dyn futures_util::task::ArcWake`
error: aborting due to previous error
For more information about this error, try `rustc --explain E0599`.
error: Could not compile `stdweb`.

The commit from in nightly that changed the Syntax: rust-lang/futures-rs#1676

[koute]

Thanks!