Only for evaluating expressions for now and right now the only exposed
operation is to debug print a value on the stack, this obviously needs to
be expanded.
I've done this for two reasons:
1. A Lua-style stack API is much nicer to work with than to manually manage
refcounts.
2. We'll soon need a more sophisticated garbage collector (if you even want
to count the refcounting as garbage collection). For this, the GC will
need root objects to start tracing for live objects, the stack will be
one of these roots.
You can now set keys in dictionaries to a value. If a key in a key path is
missing, we automatically create an empty dictionary. Otherwise setting
deeply nested keys becomes annoying.
We're still missing predicates (need to be able to call functions for that
one) and assignments into dictionaries. But we now can deconstruct a list
and check against constants.
So things like this work now:
[1 foo ~bar [a b]] = [1 "Hello" 2 3 4 [5 6]]
# foo is: "Hello"
# bar is: [2 3 4]
# a is: 5
# b is: 6
Pretty cool :)
Since we're using refcounts, we don't really copy anything and no error can
occur. So let's make these callbacks return void to simplify things. This
also makes the return value false of the value getters unambiguous: It now
always means that the key was not present.
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.
