Add a LiveRange data structure.

We will track live ranges separately for each SSA value, rather than per
virtual register like LLVM does.

This is the basis for a register allocator, so place it in a new
regalloc module.

lib/cretonne/src/regalloc/mod.rs

@@ -0,0 +1,5 @@
+//! Register allocation.
+//!
+//! This module contains data structures and algorithms used for register allocation.
+
+pub mod liverange;