Various refactorings around bignum arithmetic

src/debug.c

@@ -42,6 +42,29 @@ void debug_print( wchar_t *message, int level ) {
 #endif
 }
 
+/**
+ * stolen from https://stackoverflow.com/questions/11656241/how-to-print-uint128-t-number-using-gcc
+ */
+void debug_print_128bit( __int128_t n, int level ) {
+  #ifdef DEBUG
+  if ( level & verbosity ) {
+    if (n == 0) {
+      fwprintf(stderr, L"0");
+    } else {
+      char str[40] = {0}; // log10(1 << 128) + '\0'
+      char *s = str + sizeof(str) - 1; // start at the end
+      while (n != 0) {
+        if (s == str) return; // never happens
+
+        *--s = "0123456789"[n % 10]; // save last digit
+        n /= 10;                     // drop it
+      }
+      fwprintf(stderr, L"%s", s);
+    }
+  }
+  #endif
+}
+
 /**
  * print a line feed to stderr, if `verbosity` matches `level`.
  * `verbosity is a set of flags, see debug_print.h; so you can