Add support for FnOnce to Closure<T>

[fitzgen]

Sort of. By turning them into FnMut and doing a dynamic check that they are only ever called once.

I initially tried to get Closure<FnOnce(...) -> R> working, but that proved too difficult.

[alexcrichton]

Looks great to me! I think CI may be failing though?

[alexcrichton]

Technically this isn't quite as optimized as we might otherwise want because a FnOnce that doesn't actually close over anything will allocate because of the discriminant on Option<T>. I think fully supporting this in the limit though will require specialized support in the JS bindings (the JS bindings are the ones to clear the stored pointer).

Mind throwing a comment somewhere about that? I think this is totally fine as a first pass

[alexcrichton]

(technically the same comment for the one below too)

[fitzgen]

https://travis-ci.com/rustwasm/wasm-bindgen/jobs/179355779#L951-L961

RuntimeError: memory access out of bounds
    at dlmalloc::dlmalloc::Dlmalloc::insert_large_chunk::h752576fb10674c30 (wasm-function[1556]:244)
    at dlmalloc::dlmalloc::Dlmalloc::free::h4e7d0bccb8a2e263 (wasm-function[659]:806)
    at __rdl_dealloc (wasm-function[10159]:8)
    at __rust_dealloc (wasm-function[10043]:7)
    at alloc::alloc::dealloc::h1b984820f03b24d3 (wasm-function[4389]:100)
    at <alloc::alloc::Global as core::alloc::Alloc>::dealloc::h5015cfcc10ef6ccf (wasm-function[5536]:84)
    at <alloc::rc::Rc<T> as core::ops::drop::Drop>::drop::h6a78b25b82ddd7e0 (wasm-function[2222]:216)
    at core::ptr::real_drop_in_place::h3aa3cadebfeeb41a (wasm-function[8375]:38)
    at core::ptr::real_drop_in_place::hc4e654cf5dfc7dc7 (wasm-function[5383]:84)
    at wasm_bindgen_test::__rt::State::log_test_result::h3d6ffabb89d2894e (wasm-function[672]:841)

wat