This PR is a complete redesign of our current native functions and closures API.
I was a bit dissatisfied with our previous design (even though I created it 😆), because it had a lot of superfluous traits, a forced usage of `Gc<GcCell<T>>` and an overly restrictive `NativeObject` bound. This redesign, on the other hand, simplifies a lot our public API, with a simple `NativeCallable` struct that has several constructors for each type of required native function.
This new design doesn't require wrapping every capture type with `Gc<GcCell<T>>`, relaxes the trait requirement to `Trace + 'static` for captures, can be reused in both `JsObject` functions and (soonish) host defined functions, and is (in my opinion) a bit cleaner than the previous iteration. It also offers an `unsafe` API as an escape hatch for users that want to pass non-Copy closures which don't capture traceable types.
Would ask for bikeshedding about the names though, because I don't know if `NativeCallable` is the most precise name for this. Same about the constructor names; I added the `from` prefix to all of them because it's the "standard" practice, but seeing the API doesn't have any other method aside from `call`, it may be better to just remove the prefix altogether.
Let me know what you think :)
José Julián Espina
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