Refactor error reporting for bounds checks. (#464)

This introduces a very limited formatter that can embed strings and hex
representations of pointers / integers in an internal buffer. This is
used to format error strings for passing to `Pal::error`.  This is used,
in turn, by a wrapper for reporting bounds checks, which can be used by
external functions to implement bounds checks.

This removes the sprintf_l usage from the bounds checks.

This provides enough of a format implementation that the tests
introduced in #465 can be refactored to use this, instead of their
custom `printf` wrapper and that can be used by SNMALLOC_CHECK.  This
will be a follow-on PR.

src/test/func/malloc/malloc.cc

@@ -237,6 +237,24 @@ int main(int argc, char** argv)
 
   setup();
 
+  // Smoke test the fatal error builder.  Check that it can generate strings
+  // including all of the kinds of things that it expects to be able to format.
+  void* fakeptr = reinterpret_cast<void*>(static_cast<uintptr_t>(0x42));
+  FatalErrorBuilder<1024> b{
+    "testing pointer {} size_t {} message, {} world, null is {}",
+    fakeptr,
+    size_t(42),
+    "hello",
+    nullptr};
+  if (
+    strcmp(
+      "testing pointer 0x42 size_t 0x2a message, hello world, null is 0x0",
+      b.get_message()) != 0)
+  {
+    printf("Incorrect rendering of fatal error message: %s\n", b.get_message());
+    abort();
+  }
+
   our_free(nullptr);
 
   /* A very large allocation size that we expect to fail. */