This is analogous to dictionaries and ensures that no circular references
can be created when using the exported API in apfl.h.
This also changes apfl_value_copy into apfl_value_incref to better reflect
what it does and to reflect that it is no longer an operation that can
fail.
This avoids copying the string every time we pass it around. Not too
important right now, but will become important onve we're able to evaluate
more complex expressions.
