Our previous implementation of `toJSON()` was quite hacky.

It replaced values inside the structure with objects that had a custom
`toJSON()` serializer, and then called `JSON.stringify()` on the result.

The resulting JSON would have special markers in it where the Token
values would be string-substituted back in.

It's actually easier and gives us more control to just implement
JSONification ourselves in a Token-aware recursive function.

This change has been split off from a larger, upcoming PR in order
to make the individual reviews smaller.

…ojson