[C++] Computed instances are int32_t
Computed instances in C++ code have types `int32_t`, which breaks the code needing them to be `uint64_t` and is inefficient for the code for which `uint16_t` is enough.

So we need
1. set them to int64_t as a quick fix
2. implement #127 
3. implement derivation of the sizes based on operations used and types of operands and/or usages of the result. More precisely, everything used as an array index must be unsigned and everything used in `pos` must be `size_t`.
