Add C++ demangling to privileged runtime

Cosmo will now print C++ symbols correctly in --ftrace logs and
backtraces. Doing this required reducing the memory requirement
of the __demangle() function by 3x. This was accomplished using
16-bit indices and 16-bit malloc granularity. That puts a limit
on the longest symbol we can successfully decode, which I think
would be around 6553 characters long, given a 65536-byte buffer

examples/demangle.c

@@ -0,0 +1,67 @@
+#include <assert.h>
+#include <cxxabi.h>
+#include <errno.h>
+#include <stdckdint.h>
+#include <stdio.h>
+#include <string.h>
+#include "libc/assert.h"
+#include "libc/calls/struct/timespec.h"
+#include "libc/cosmo.h"
+#include "libc/runtime/runtime.h"
+#include "libc/stdio/internal.h"
+#include "libc/stdio/stdio.h"
+#include "libc/str/str.h"
+#include "libc/time.h"
+
+void nothing(void) {
+}
+
+void (*barrier)(void) = nothing;
+
+struct timespec now(void) {
+  struct timespec ts;
+  timespec_get(&ts, TIME_UTC);
+  return ts;
+}
+
+long long tonanos(struct timespec x) {
+  return x.tv_sec * 1000000000ll + x.tv_nsec;
+}
+
+struct timespec tub(struct timespec a, struct timespec b) {
+  a.tv_sec -= b.tv_sec;
+  if (a.tv_nsec < b.tv_nsec) {
+    a.tv_nsec += 1000000000;
+    a.tv_sec--;
+  }
+  a.tv_nsec -= b.tv_nsec;
+  return a;
+}
+
+#define BENCH(ITERATIONS, WORK_PER_RUN, CODE)                              \
+  do {                                                                     \
+    struct timespec start = now();                                         \
+    for (int i = 0; i < ITERATIONS; ++i) {                                 \
+      barrier();                                                           \
+      CODE;                                                                \
+    }                                                                      \
+    long long work = WORK_PER_RUN * ITERATIONS;                            \
+    double nanos = (tonanos(tub(now(), start)) + work - 1) / (double)work; \
+    printf("%10g ns %2dx %s\n", nanos, ITERATIONS, #CODE);                 \
+  } while (0)
+
+char got[5632];
+char huge[262144];
+
+__static_yoink("PrintBacktraceUsingSymbols");
+__static_yoink("GetSymbolTable");
+
+int main(int argc, char *argv[]) {
+  ShowCrashReports();
+  const char *sym =
+      "_ZN12_GLOBAL__N_116tinyBLAS_Q0_AVX2ILi0E10block_q4_010block_"
+      "q8_0fE4gemmILi3ELi2ELi0EEEvllll";
+  __demangle(got, sym, sizeof(got));
+  printf("%s\n", got);
+  BENCH(1000, 1, __demangle(got, sym, sizeof(got)));
+}