Make old C++ demangler asynchronous signal safe

It's now possible to safely print C++ backtraces from signal handlers.
This symbol demangler doesn't need malloc, tls, or even static memory.
Additionally, this change makes it 2x faster and adds test cases. It's
almost as performant and accurate as the libcxxabi implementation now.

libc/runtime/isheap.c

@@ -29,7 +29,7 @@
  * @assume stack memory isn't stored beneath %rsp (-mno-red-zone)
  * @deprecated
  */
-optimizesize bool32 _isheap(void *p) {
+optimizesize bool32 _isheap(const void *p) {
   intptr_t x, y;
   x = kAutomapStart;
   y = x + kAutomapSize;