runtime (gc): simplify extalloc and correct GC scan bounds

This change replaces the overcomplicated treap system used previously by the extalloc GC.
The treap was difficult to understand, and added a lot of complexity.
It has been replaced by a simple sorted list, which has roughly the same space overhead and time complexity.

Additionally, a bug was found in the termination condition for the scanning code for both collectors.
On the extalloc collector, this would cause a trailing pointer to be missed.
On the conservative collector, this would cause the scanner to read out of bounds on architectures where pointer size was greater than alignment (AVR mostly).

Author: niaow

src/runtime/gc_conservative.go

@@ -410,7 +410,7 @@ func markRoots(start, end uintptr) {
 		}
 	}
 
-	for addr := start; addr < end; addr += unsafe.Alignof(addr) {
+	for addr := start; addr+unsafe.Sizeof(unsafe.Pointer(nil)) <= end; addr += unsafe.Alignof(addr) {
 		root := *(*uintptr)(unsafe.Pointer(addr))
 		markRoot(addr, root)
 	}