If you assign into a member access (`foo.bar = baz` or `foo@bar = baz`), it
is no longer permitted that the LHS of the at/dot is an arbitrary
assignable. It now must be a variable, at or dot. This disallows some silly
constructs (e.g. `[foo]@bar = baz`), increases the similarity to function
parameters and should make writing the evaluation code for these more easy.
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.
Increasing the refcount (confusingly called copy before, fixed that too)
didn't work properly, as the object was copied and the refcount was only
updated in one of the copies.
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.
