Remove `allocator` field in `Deserializer`

[ibokuri]

I think the initial reasoning for the field was so that deserialization and visitor-related operations could have different allocators. But I'm not even sure if that makes sense or is a good use-case.

~Plus, in deserializeString and deserializeEnum, I think it makes more sense if the visitor duplicates the passed-in string instead of the deserializer since the allocated string ends up being either the value produced by the visitor or a part of the produced value.~