Use after free in Neon external buffers

Summary

Neon provides functionality for creating JavaScript `ArrayBuffer` (and the `Buffer` subtype) instances backed by bytes allocated outside of V8/Node. The [`JsArrayBuffer::external`](https://docs.rs/neon/0.10.0/neon/types/struct.JsArrayBuffer.html#method.external) and [`JsBuffer::external`](https://docs.rs/neon/0.10.0/neon/types/struct.JsBuffer.html#method.external) did not require `T: 'static` prior to Neon `0.10.1`. This allowed creating an externally backed buffer from types that may be freed while they are still referenced by a JavaScript `ArrayBuffer`. The following example demonstrates use after free. It compiles on versions `<0.10.1` and fails to compile afterward. ```rust pub fn soundness_hole(mut cx: FunctionContext) -> JsResult<JsArrayBuffer> { let mut data = vec![0u8, 1, 2, 3]; // Creating an external from `&mut [u8]` instead of `Vec<u8>` since there is a blanket impl // of `AsMut<T> for &mut T` let buf = JsArrayBuffer::external(&mut cx, data.as_mut_slice()); // `buf` is still holding a reference to `data`! drop(data); Ok(buf) } ```