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/meta/gen_legalizer.py

@@ -355,7 +355,7 @@ def gen_xform_group(xgrp, fmt, type_sets):
             'cfg: &mut ::flowgraph::ControlFlowGraph, '
             'pos: &mut ir::Cursor) -> '
             'bool {{'.format(xgrp.name), '}'):
-        fmt.line('use ir::InstBuilder;')
+        fmt.line('use ir::{InstBuilder, CursorBase};')
 
         # Gen the instruction to be legalized. The cursor we're passed must be
         # pointing at an instruction.