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/print_errors.rs

@@ -0,0 +1,33 @@
+//! Utility routines for pretty-printing error messages.
+
+use ir;
+use verifier;
+use result::CtonError;
+use isa::TargetIsa;
+use std::fmt::Write;
+
+/// Pretty-print a verifier error.
+pub fn pretty_verifier_error(
+    func: &ir::Function,
+    isa: Option<&TargetIsa>,
+    err: verifier::Error,
+) -> String {
+    let mut msg = err.to_string();
+    match err.location {
+        ir::entities::AnyEntity::Inst(inst) => {
+            write!(msg, "\n{}: {}\n\n", inst, func.dfg.display_inst(inst, isa)).unwrap()
+        }
+        _ => msg.push('\n'),
+    }
+    write!(msg, "{}", func.display(isa)).unwrap();
+    msg
+}
+
+/// Pretty-print a Cretonne error.
+pub fn pretty_error(func: &ir::Function, isa: Option<&TargetIsa>, err: CtonError) -> String {
+    if let CtonError::Verifier(e) = err {
+        pretty_verifier_error(func, isa, e)
+    } else {
+        err.to_string()
+    }
+}