Use Box<str> instead of Box<String> for UnscannedTextFragmentInfo

This removes an extraneous layer of indirection, without increasing the size
of the `SpecificFragmentInfo` enum (because `Box<str>` is a fat pointer, which
is the same size as the `FlowRef` trait object that is already stored in
`InlineAbsoluteHypotheticalFragmentInfo`)

components/layout/lib.rs

@@ -4,6 +4,7 @@
 
 #![feature(append)]
 #![feature(arc_unique)]
+#![feature(box_str)]
 #![feature(box_syntax)]
 #![feature(cell_extras)]
 #![feature(custom_derive)]