Export box_region_join into FFI
There's currently a number of region allocator functions exported, but they don't work very well if you don't know ahead of time the total size of allocations. region_join is a function which exists exactly for that reason, so it should also be exported to the FFI
This is needed to simplify the ugly RegionBuffer code in picodata (see picodata/picodata/picodata!973 (comment 84628))