Add internal sprintf-compatible format function

format() has full support for sprintf format but is written completely in D.
It is currently internal, since it is not typesafe and uses GC at one place.
After some work the function can be made public.

source/tanya/format/conv.d

@@ -613,12 +613,14 @@ private @nogc unittest
 }
 
 // Returns the last part of buffer with converted number.
-package char[] number2String(T)(const T number, out char[20] buffer)
+package(tanya) char[] number2String(T)(const T number,
+                                       return ref char[21] buffer)
+if (isIntegral!T)
 {
     // abs the integer.
     ulong n64 = number < 0 ? -cast(long) number : number;
 
-    char* start = buffer.ptr + buffer.sizeof;
+    char* start = buffer[].ptr + buffer.sizeof - 1;
 
     while (true)
     {
@@ -645,7 +647,8 @@ package char[] number2String(T)(const T number, out char[20] buffer)
         // Ignore the leading zero if it was the last part of the integer.
         if (n64 == 0)
         {
-            if ((start[0] == '0') && (start != (buffer.ptr + buffer.sizeof)))
+            if ((start[0] == '0')
+             && (start != (buffer[].ptr + buffer.sizeof -1)))
             {
                 ++start;
             }
@@ -660,7 +663,7 @@ package char[] number2String(T)(const T number, out char[20] buffer)
     }
 
     // Get the length that we have copied.
-    uint l = cast(uint) ((buffer.ptr + buffer.sizeof) - start);
+    uint l = cast(uint) ((buffer[].ptr + buffer.sizeof - 1) - start);
     if (l == 0)
     {
         *--start = '0';
@@ -672,12 +675,13 @@ package char[] number2String(T)(const T number, out char[20] buffer)
         ++l;
     }
 
-    return buffer[$ - l .. $];
+    return buffer[$ - l - 1 .. $ - 1];
 }
 
+// Converting an integer to string.
 private pure nothrow @system @nogc unittest
 {
-    char[20] buf;
+    char[21] buf;
 
     assert(number2String(80, buf) == "80");
     assert(number2String(-80, buf) == "-80");