fix(neon): Relax lifetime constraints on With and provide helper for forcing HRTB with JS values

[kjvalencik]

With is primarily intended as a helper for providing a thunk executed on the main thread after an asynchronous operation. However, it's not currently possible to return JavaScript values due to unnecessarily restrictive lifetimes.

// DOES NOT COMPILE
use neon::types::extract::{TryIntoJs, With};

#[neon::export(task)]
fn example() -> impl for<'cx> TryIntoJs<'cx> {
    With(|cx| cx.number(42.0))
}

These changes relax thoe bounds, but unfortunately causes Rust to no longer correctly infer the correct HRTB. As a workaround, an extract::with helper is provided.

use neon::types::extract::{self, TryIntoJs};

// Compiles :)
#[neon::export(task)]
fn example() -> impl for<'cx> TryIntoJs<'cx> {
    extract::with(|cx| Ok(cx.number(42.0)))
}

I could not find a way to make a helper that works with both JsResult and Rust data without a lifetime. Since someone returning Rust data always has the option to call .try_into_js(cx), I chose to keep the API simple by only providing the one helper.

Open Questions

• Is it possible to implement a helper that supports generic return types of O: TryIntoJs<'cx> that aren't 'static?
• Should we make With private? The with type would return an opaque impl for<'cx> TryIntoJs<'cx>. This could simplify the API.