Revert "No form switching.", make it take a bit longer.

You can't transform while in lich form in the first place, and it's the only
one with very deadly downsides.  Shedding slowness or rPois- is a milder abuse,
and even worse, this change made the interface painful.

Instead, let's have the transforming action take a bit more time.

This reverts commit cafb36c.

crawl-ref/source/transform.cc

@@ -604,13 +604,6 @@ bool transform(int pow, transformation_type which_trans, bool force,
         return (false);
     }
 
-    // Don't allow changing form. Must manually untransform first.
-    if (!force && you.form != TRAN_NONE && you.form != which_trans)
-    {
-        mpr("You must return to your normal form first.");
-        return false;
-    }
-
     if (!_transformation_is_safe(which_trans, env.grid(you.pos()), force))
         return (false);
 
@@ -797,6 +790,10 @@ bool transform(int pow, transformation_type which_trans, bool force,
     if (just_check)
         return (true);
 
+    // Switching between forms takes a bit longer.
+    if (!force && previous_trans != TRAN_NONE && previous_trans != which_trans)
+        you.time_taken = div_rand_round(you.time_taken * 3, 2);
+
     // All checks done, transformation will take place now.
     you.redraw_quiver       = true;
     you.redraw_evasion      = true;