Use uniform biases in AnchorMap, AnchorRangeMap

Specifying a different bias for every point makes the interface feel pretty unwieldy and we don't really use it.

crates/buffer/src/anchor.rs

@@ -19,7 +19,8 @@ pub struct Anchor {
 #[derive(Clone)]
 pub struct AnchorMap<T> {
     pub(crate) version: clock::Global,
-    pub(crate) entries: Vec<((FullOffset, Bias), T)>,
+    pub(crate) bias: Bias,
+    pub(crate) entries: Vec<(FullOffset, T)>,
 }
 
 #[derive(Clone)]