Add iota/ltoa implementation

Borrowed from SAMD core; fixes #6

cores/arduino/itoa.c

@@ -0,0 +1,126 @@
+/*
+  Copyright (c) 2014 Arduino LLC.  All right reserved.
+
+  This library is free software; you can redistribute it and/or
+  modify it under the terms of the GNU Lesser General Public
+  License as published by the Free Software Foundation; either
+  version 2.1 of the License, or (at your option) any later version.
+
+  This library is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+  See the GNU Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public
+  License along with this library; if not, write to the Free Software
+  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+*/
+
+#include <api/itoa.h>
+#include <string.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+extern char* itoa( int value, char *string, int radix )
+{
+  return ltoa( value, string, radix ) ;
+}
+
+extern char* ltoa( long value, char *string, int radix )
+{
+  char tmp[33];
+  char *tp = tmp;
+  long i;
+  unsigned long v;
+  int sign;
+  char *sp;
+
+  if ( string == NULL )
+  {
+    return 0 ;
+  }
+
+  if (radix > 36 || radix <= 1)
+  {
+    return 0 ;
+  }
+
+  sign = (radix == 10 && value < 0);
+  if (sign)
+  {
+    v = -value;
+  }
+  else
+  {
+    v = (unsigned long)value;
+  }
+
+  while (v || tp == tmp)
+  {
+    i = v % radix;
+    v = v / radix;
+    if (i < 10)
+      *tp++ = i+'0';
+    else
+      *tp++ = i + 'a' - 10;
+  }
+
+  sp = string;
+
+  if (sign)
+    *sp++ = '-';
+  while (tp > tmp)
+    *sp++ = *--tp;
+  *sp = 0;
+
+  return string;
+}
+
+extern char* utoa( unsigned int value, char *string, int radix )
+{
+  return ultoa( value, string, radix ) ;
+}
+
+extern char* ultoa( unsigned long value, char *string, int radix )
+{
+  char tmp[33];
+  char *tp = tmp;
+  long i;
+  unsigned long v = value;
+  char *sp;
+
+  if ( string == NULL )
+  {
+    return 0;
+  }
+
+  if (radix > 36 || radix <= 1)
+  {
+    return 0;
+  }
+
+  while (v || tp == tmp)
+  {
+    i = v % radix;
+    v = v / radix;
+    if (i < 10)
+      *tp++ = i+'0';
+    else
+      *tp++ = i + 'a' - 10;
+  }
+
+  sp = string;
+
+
+  while (tp > tmp)
+    *sp++ = *--tp;
+  *sp = 0;
+
+  return string;
+}
+
+#ifdef __cplusplus
+} // extern "C"
+#endif