big arrays [was Re: bug found]

[Stephen Weeks]

> What would be your proposed object layout (or only arrays having top-bit-0)?

Header word
	         mark
	31 30 29 28
normal   1  0  0
stack    1  1  0
cont     1  1  1
array    0  0  0

As now, non-arrays are preceeded by a single header word, while arrays
are preceeded by a header word, which is preceeded by a length.

Bits 0 - 27 are used in normal and array headers as they are now, to
indicate the number of bytes of nonpointers and the number of
pointers.