Add new whole heap verifier that can catch a whole lot more problems than previous...

Add new whole heap verifier that can catch a whole lot more problems than previous tools did.

	* sgen-gc.c Add verify-before-allocs and verify-before-collections experimental
	options to perform heap verification at the many different points in the app.

	This does check all pointers in the heap, so it's much much slower than
	the previous option.