don't erase ptr type early when construction a Cons HeapCellValue

don't erase ptr type early when construction a Cons HeapCellValue

rather than passing an address as usize pass the ArenaHeader pointer
similarly don't return a u8 ptr but use a ArenaHeader pointer instead

Don't convert the pointer to a ConsPtr by going through native endian
bytes in between.
We are exploiting the fact that the 3 least significant bytes are zero
for pointer to types of alignment 8 and we expect these to line up with
the f, m, and tag field at the end of the ConsPtr struct, but using
native endiannes for this would only work on big endian systems.