Exclusive data subset endpoints #90
Description
Currently, Fenix data subsets cover the range [begin, end], while most programming constructs operate with an exclusive end: [begin, end)
In other Fenix functions paramaters, we use an exclusive end like normal. For example, the length value passed in to Fenix_Data_member_store is exclusive - we will not try to store a value at buffer[length]. However, to make the equivalent data subset a value of length-1 must be passed.
This is an easy source of memory access errors, so we should be more consistent and use exclusive endpoints for data subsets