Expose buf_index (#187)

This makes it easier to build abstractions on top of the `FixedBufferRegistry`.

Author: Nehliin

src/buf/fixed/handle.rs

@@ -55,7 +55,8 @@ impl FixedBuf {
         }
     }
 
-    pub(crate) fn buf_index(&self) -> u16 {
+    /// Index of the underlying registry buffer
+    pub fn buf_index(&self) -> u16 {
         self.index
     }
 }