buttons should be more mixin friendly

less/buttons.less

@@ -18,32 +18,32 @@
   white-space: nowrap;
   .button-size(@padding-base-vertical; @padding-base-horizontal; @font-size-base; @line-height-base; @border-radius-base);
   .user-select(none);
-}
 
-.btn:focus {
+  &:focus {
     .tab-focus();
-}
+  }
 
-.btn:hover,
-.btn:focus {
+  &:hover,
+  &:focus {
     color: @btn-default-color;
     text-decoration: none;
-}
+  }
 
-.btn:active,
-.btn.active {
+  &:active,
+  &.active {
     outline: 0;
     background-image: none;
     .box-shadow(inset 0 3px 5px rgba(0,0,0,.125));
-}
+  }
 
-.btn.disabled,
-.btn[disabled],
-fieldset[disabled] .btn {
+  &.disabled,
+  &[disabled],
+  fieldset[disabled] & {
     cursor: not-allowed;
     pointer-events: none; // Future-proof disabling of clicks
     .opacity(.65);
     .box-shadow(none);
+  }
 }