Use ouroboros in FontSystem to avoid lifetime bound

Perhaps not quite what ouroboros is expected to be used for, but it's
not too bad, and avoiding the lifetime bound can be a huge help.

src/buffer.rs

@@ -139,7 +139,7 @@ impl fmt::Display for Metrics {
 
 /// A buffer of text that is shaped and laid out
 pub struct Buffer<'a> {
-    font_system: &'a FontSystem<'a>,
+    font_system: &'a FontSystem,
     /// Lines (or paragraphs) of text in the buffer
     pub lines: Vec<BufferLine>,
     metrics: Metrics,
@@ -153,7 +153,7 @@ pub struct Buffer<'a> {
 impl<'a> Buffer<'a> {
     /// Create a new [Buffer] with the provided [FontSystem] and [Metrics]
     pub fn new(
-        font_system: &'a FontSystem<'a>,
+        font_system: &'a FontSystem,
         metrics: Metrics,
     ) -> Self {
         let mut buffer = Self {