Move library crates under 'lib/'.

Give these crates each a more standard directory layout with sources in
a 'src' sub-sirectory and Cargo.toml in the top lib/foo directory.

Add license and description fields to each.

The build script for the cretonne crate now lives in
'lib/cretonne/build.rs' separating it from the normal library sources
under 'lib/cretonne/src'.

lib/cretonne/src/lib.rs

@@ -0,0 +1,25 @@
+
+// ====------------------------------------------------------------------------------------==== //
+//
+// Cretonne code generation library.
+//
+// ====------------------------------------------------------------------------------------==== //
+
+pub use verifier::verify_function;
+pub use write::write_function;
+pub use legalizer::legalize_function;
+
+pub const VERSION: &'static str = env!("CARGO_PKG_VERSION");
+
+pub mod ir;
+pub mod isa;
+pub mod cfg;
+pub mod dominator_tree;
+pub mod entity_map;
+pub mod settings;
+pub mod verifier;
+
+mod write;
+mod constant_hash;
+mod predicates;
+mod legalizer;