make ICL_IMPL_SPACE configurable, to be able to use external sets (e.g. abseil btree_set)

use something like:
```
#if defined(ICL_USE_BOOST_MOVE_IMPLEMENTATION)
#   define ICL_IMPL_SPACE boost::container
#elif defined(ICL_USE_STD_IMPLEMENTATION)
#   define ICL_IMPL_SPACE std
#elif !defined(ICL_IMPL_SPACE)
#   define ICL_IMPL_SPACE std
#endif
```
in impl_config.hpp