Change da macros to use .

[jonathandw743]

Changed da macros to use . instead of ->.
The da macros should be able to be used on values and pointers (which they currently are) using:
arena_da_append(a, da, item) where da is a pointer, which expands to code using (da)->capacity and
arena_da_append(a, &da, item) where da is a value, which expands to code using (&da)->capacity
(da)->capacity is good
(&da)->capacity is not good because it is usually optimised to da.capacity, but this is not guaranteed.
But, in this PR, the calling code would use:
arena_da_append(a, *da, item) where da is a pointer, which expands to code using (*da).capacity and
arena_da_append(a, da, item) where da is a value, which expands to code using (da).capacity
(*da).capacity is good because it is equivalent to da->capacity
(da).capacity is good