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/cosmo.h

@@ -6,6 +6,8 @@ errno_t cosmo_once(_Atomic(uint32_t) *, void (*)(void));
 int systemvpe(const char *, char *const[], char *const[]) libcesque;
 char *GetProgramExecutableName(void);
 void unleaf(void);
+int __demangle(char *, const char *, size_t);
+int __is_mangled(const char *);
 
 COSMOPOLITAN_C_END_
 #endif /* COSMOPOLITAN_LIBC_COSMO_H_ */

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;

libc/runtime/runtime.h

@@ -110,9 +110,9 @@ void *_mapanon(size_t) attributeallocsize((1)) mallocesque;
 void *_mapshared(size_t) attributeallocsize((1)) mallocesque;
 void CheckForMemoryLeaks(void);
 void CheckForFileLeaks(void);
+bool32 _isheap(const void *);
 void __enable_threads(void);
 void __oom_hook(size_t);
-bool32 _isheap(void *);
 /* code morphing */
 void __morph_begin(void);
 void __morph_end(void);