Move most Cursor methods into a CursorBase trait.

The Cursor navigation methods all just depend on the cursor's position
and layout reference. Make a CursorBase trait that provides access to
this information with methods and implement the navigation methods on
top of that.

This makes it possible to have multiple types implement the cursor
interface.

lib/cretonne/src/binemit/relaxation.rs

@@ -28,7 +28,7 @@
 //! ```
 
 use binemit::CodeOffset;
-use ir::{Function, DataFlowGraph, Cursor, InstructionData, Opcode, InstEncodings};
+use ir::{Function, DataFlowGraph, Cursor, CursorBase, InstructionData, Opcode, InstEncodings};
 use isa::{TargetIsa, EncInfo};
 use iterators::IteratorExtras;
 use result::CtonError;