Move EntityList and SparseMap into the entity module.

These data structures are dependent on EntityRef and EntityMap, so it
makes sense to keep them in the same module.

lib/cretonne/src/regalloc/liveness.rs

@@ -175,13 +175,13 @@
 //!
 //! There is some room for improvement.
 
+use entity::SparseMap;
 use flowgraph::ControlFlowGraph;
 use ir::dfg::ValueDef;
 use ir::{Function, Value, Inst, Ebb, Layout, ProgramPoint};
 use isa::{TargetIsa, EncInfo};
 use regalloc::affinity::Affinity;
 use regalloc::liverange::LiveRange;
-use sparse_map::SparseMap;
 use std::mem;
 use std::ops::Index;