//
// When a mspan is allocated, state == mSpanInUse or mSpanManual
// and heapmap(i) == span for all s->start <= i < s->start+s->npages.

// Every mspan is in one doubly-linked list, either in the mheap's
// busy list or one of the mcentral's span lists.

// An mspan representing actual memory has state mSpanInUse,
// mSpanManual, or mSpanFree. Transitions between these states are
// constrained as follows:
//