Type varsize should be mapped to size_t in C++

[mikir]

Normally, varsize type should be mapped into native size_t type in C++.

Unfortunately, this type has different size in 32-bit and 64-bit C++. But C++ generator needs to know bit size of all native types to be able to generate corresponded range check code. Currently C++ generator does not know if generated code is aimed for 32-bit or 64-bit, so it can't generate corresponded range check code for size_t values. This should be solved somehow in the future.