Fix more recursive failures

[CUB3D]

This applies the same changes as in #67 to all other Value types that can have non-trivial children leading to reference cycles.

Also adds a fuzzer for the parse_single_element function. With this changeset neither fuzz_amf0_body fuzz_amf3_body or fuzz_amf3_single_element panic after ~500million fuzz cases each.

Note this is not as clean as I would like, but I want to fix the panics first and worry about the refactor later

[torokati44]

Thank you for addressing my nitpicks!
As this is still mostly your project, I trust that you know what you're doing; and quickly glancing through the changes, I don't see anything immediately problematic.
While I would've kept the commit history neater, it's not a big deal (and this repo already has many "cosmetic" commits) - so in the name of progress, I say let's just get this in.