limit checks

[Henry Cejtin]

Won't this (checking that limit checks are for exactly the sum of the sizes of
all types being allocated) prevent putting in an optimization which checks for
more than that so that we have fewer limit checks?  I'm thinking of something
like
	check for atleast A+B+C bytes available
	allocate A
	if condition
		allocate B
	allocate C
Collapsing this to one limit check would be nice (although you have to be
careful to maintain safe-for-space.