Add syntax sugar map + RefTuple combination.

`map` on top of ndslice composed of `RefTuples` recognises it and
passes list of args instead of single tuple.

Old:
```
    auto c = cartesian(a, b)
        .map!"a.a + a.b";
```

New:
```
    auto c = cartesian(a, b)
        .map!"a + b";
```

Author: 9il

source/mir/functional.d

@@ -115,7 +115,7 @@ RefTuple!Args tuple(Args...)(auto ref Args args)
 }
 
 /// Removes $(LREF Ref) shell.
-T unref(V : Ref!T, T)(V value)
+ref T unref(V : Ref!T, T)(V value)
 {
     return *value.__ptr;
 }

source/mir/ndslice/field.d

@@ -53,9 +53,17 @@ struct MapField(Field, alias fun)
     +/
     static alias __map(alias fun1) = MapField__map!(Field, fun, fun1);
 
-    auto ref opIndex(T...)(T index)
+    auto ref opIndex(T...)(auto ref T index)
     {
-        return fun(_field[index]);
+        import mir.functional: RefTuple, unref;
+        static if (is(typeof(_field[index]) : RefTuple!K, K...))
+        {
+            import mir.ndslice.field: _iotaArgs; 
+            auto t = _field[index];
+            return mixin("fun(" ~ _iotaArgs!(K.length, "t.expand[", "].unref, ") ~ ")");
+        }
+        else
+            return fun(_field[index]);
     }
 
     auto length()() @property

source/mir/ndslice/iterator.d

@@ -508,8 +508,15 @@ struct MapIterator(Iterator, alias fun)
 
     auto ref opUnary(string op : "*")()
     {
+        import mir.functional: RefTuple, unref;
         static if (is(Iterator : ZipIterator!(Iterators), Iterators...))
             return mixin("fun(" ~ _iotaArgs!(Iterators.length, "*_iterator._iterators[", "], ") ~ ")");
+        else
+        static if (is(typeof(*_iterator) : RefTuple!T, T...))
+        {
+            auto t = *_iterator;
+            return mixin("fun(" ~ _iotaArgs!(T.length, "t.expand[", "].unref, ") ~ ")");
+        }
         else
             return fun(*_iterator);
     }
@@ -520,8 +527,15 @@ struct MapIterator(Iterator, alias fun)
 
     auto ref opIndex()(ptrdiff_t index)
     {
+        import mir.functional: RefTuple, unref;
         static if (is(Iterator : ZipIterator!(Iterators), Iterators...))
             return mixin("fun(" ~ _iotaArgs!(Iterators.length, "_iterator._iterators[", "][index], ") ~ ")");
+        else
+        static if (is(typeof(_iterator[0]) : RefTuple!T, T...))
+        {
+            auto t = _iterator[index];
+            return mixin("fun(" ~ _iotaArgs!(T.length, "t.expand[", "].unref, ") ~ ")");
+        }
         else
             return fun(_iterator[index]);
     }

source/mir/ndslice/topology.d

@@ -1749,7 +1749,7 @@ unittest
         [tuple(2.00, 0.00), tuple(2.00, 0.5), tuple(2.00, 1.0)],
         ]);
 
-    assert(s.map!"a.a * a.b" == [
+    assert(s.map!"a * b" == [
         [0.0, 0.500, 1.00],
         [0.0, 0.625, 1.25],
         [0.0, 0.750, 1.50],
@@ -2564,7 +2564,7 @@ unittest
     auto b = [ 1,  2,  3];
 
     auto c = cartesian(a, b)
-        .map!"a.a + a.b";
+        .map!"a + b";
 
     assert(c == [
         [11, 12, 13],
@@ -2579,7 +2579,7 @@ unittest
     auto b = iota([2, 3], 1);
 
     auto c = cartesian(a, b)
-        .map!"a.a + a.b";
+        .map!"a + b";
 
     assert(c.shape == [3, 2, 3]);
 
@@ -2606,7 +2606,7 @@ unittest
     auto w = [1, 2];
 
     auto c = cartesian(u, v, w)
-        .map!"a.a + a.b + a.c";
+        .map!"a + b + c";
 
     assert(c.shape == [2, 3, 2]);