refactor: move DataValue from cranelift-reader to cranelift-codegen

This is no change to functionality; the move is necessary in order to return InstructionData immediates in a structure way (see next commit).

cranelift/codegen/src/lib.rs

@@ -71,6 +71,7 @@ pub use cranelift_entity as entity;
 pub mod binemit;
 pub mod cfg_printer;
 pub mod cursor;
+pub mod data_value;
 pub mod dbg;
 pub mod dominator_tree;
 pub mod flowgraph;