Add BytePointer to help with pointer conversions

This behaves mostly like a pointer, but pointer maths works in bytes,
not the native object size.

It avoids casting to char* and back when doing byte-based pointer math,
making the code easier to read.

Change-Id: I6a8681a398345f0c7d419a2cfe7244d972ffa62f
Reviewed-on: https://review.haiku-os.org/c/1086
Reviewed-by: waddlesplash <waddlesplash@gmail.com>