Fix limitations on vector length

[Roland Orre]

Some of our code using multidimensional arrays behaved buggy and that
showed to be caused by the limitations of vector lengths to 24 bits,
we need at least a few bits more at the moment.

Problems:
1) the allocation of the array did not cause an error message.
2) we need an urgent simple fix for utilizing big arrays.

What we inted to do is to change the SCM_LENGTH macro and the
memory allocation for vectors and arrays so when the length
need more than 24 bits the length is stored in the beginning
of the array memory, that is we add an offset of four to the
base pointer.

Any comments?

        Roland Orre