Simplified std.array.join with separator.

Previously, std.array.join with a separator would check whether it needed to add the separator on each iteration of the loop. With this change, the check is unnecessary because the first element is appended before the loop, then the loop just adds the separator and next element each iteration.

This improves the code significantly in the case of a non-forward RoR because we don't need to know the length anymore. It's simpler, and will be slightly faster.

std/array.d

@@ -1511,18 +1511,16 @@ ElementEncodingType!(ElementType!RoR)[] joinImpl(RoR, R)(RoR ror, R sep)
                           sepArr.length * (ror.length - 1);
     auto result = new Unqual!RetElem[](resultLen);
 
-    size_t i = 0;
-    size_t j = 0;
+    auto first = ror.front;
+    result[0 .. first.length] = first[];
+    size_t i = first.length;
+    ror.popFront();
     foreach(r; ror)
     {
+        result[i .. i + sepArr.length] = sepArr[];
+        i += sepArr.length;
         result[i .. i + r.length] = r[];
         i += r.length;
-
-        if(++j < ror.length)
-        {
-            result[i .. i + sepArr.length] = sepArr[];
-            i += sepArr.length;
-        }
     }
 
     return cast(RetType)result;
@@ -1539,30 +1537,14 @@ ElementEncodingType!(ElementType!RoR)[] joinImpl(RoR, R)(RoR ror, R sep)
         return typeof(return).init;
 
     auto result = appender!(typeof(return))();
-
-    static if(isForwardRange!RoR)
-    {
-        immutable numRanges = walkLength(ror);
-        size_t j = 0;
-    }
-
-    foreach(r; ror)
+    result.put(ror.front);
+    ror.popFront();
+    foreach (r; ror)
     {
+        result.put(sep);
         result.put(r);
-
-        static if(isForwardRange!RoR)
-        {
-            if(++j < numRanges)
-                result.put(sep);
-        }
-        else
-            result.put(sep);
     }
-
-    static if(isForwardRange!RoR)
-        return result.data;
-    else
-        return result.data[0 .. $ - sep.length];
+    return result.data;
 }
 
 ElementEncodingType!(ElementType!RoR)[] joinImpl(RoR)(RoR ror)