Implement `replacer` equivalent in `stringify`

[Zulu-Inuoe]

Using JSON.stringify in JS, there is a replacer function that can be provided. This serves for as a point-of-serialize coercion of values and offers an alternative way to handle the hard problem of coercing to JSON.

I think it'd be a good idea to implement here. For the case of excluding keys, I've settled on the behaviour of

1. Exclude the field - nil
2. Include the field - t
3. Include the field, and use a replacement value (values t "replacement")

[fstamour]

Can this be closed now? If not, what's left?

[Zulu-Inuoe]

@fstamour congrats I just closed it w/ 923fd268d24d799c6e8b3ea98a92c069fcfeae39 which I think was the last remaining edge-case? There might be some other weird case but future people will have to deal with it