Move the filetest harness into its own crate.

This allows us to run the tests via a library call rather than just
as a command execution. And, it's a step toward a broader goal, which
is to keep the code in the top-level src directory minimal, with
important functionality exposed as crates.

lib/cretonne/src/lib.rs

@@ -27,6 +27,7 @@ pub mod ir;
 pub mod isa;
 pub mod loop_analysis;
 pub mod packed_option;
+pub mod print_errors;
 pub mod result;
 pub mod settings;
 pub mod timing;