Change functions from pointers to handles

## Problem
- currently we de-reference all handles from the storageAPI immediately
- this works against the idea of us using the handle, so that the underlying address can be change by the storage system

## Ideas
1. change ALL functions, to work on handles instead of pointers
- this needs to be done, until we work on actual elements (i.e. arithmetic)
2. introduce a "de-reference" checkpoint (i.e. at the beginning of each epoch), where all handles are de-referenced
- this needs to be done for all handles introduced by reserveMemory()
3. introduce "doubled" API (similar to arithmetic + arithmetic with Instruction Counter)