Many internal structures refer to trace positions; this is an important type of values. In the code, it is sometimes `u32`, sometimes `usize`. While such variables usually have "_pos" suffix, having a dedicated type does improve code with type guarantees.
As `wasm32-wasi` target doesn't seem to support sanitizers, we are to
execute tests with native code. The feature `test_with_native_code` is
introduced for that.
Closes#247.
