Add a LAyout Cursor data structure.

A layout cursor can be used instead of an iterator to keep track of a
position in a basic block *and* permitting instructions to be
manipulated.

src/libcretonne/ir/mod.rs

@@ -22,6 +22,6 @@ pub use ir::instructions::{Opcode, InstructionData, VariableArgs};
 pub use ir::stackslot::StackSlotData;
 pub use ir::jumptable::JumpTableData;
 pub use ir::dfg::{DataFlowGraph, ValueDef};
-pub use ir::layout::Layout;
+pub use ir::layout::{Layout, Cursor};
 pub use ir::function::Function;
 pub use ir::builder::Builder;