Rename heim_base_atomic_max to heim_base_atomic_integer_max (ditto with _type)
in order to better reflect their usage, now we have a separate
heim_base_atomic() macro for making an atomic version of any type.
heim_base_exchange_32() and heim_base_exchange_64() inline functions for
platforms without atomics were missing (these are very inefficient but,
clearly rarely used given the lack of build error reports)
