Make functions cloneable for testing.

It's not super fast to clone a function, but it is faster than
re-parsing the test case file it came from.

Some tests want to mutate the function, and there may be other tests in
the same script that need the original function.

src/libcretonne/ir/jumptable.rs

@@ -12,6 +12,7 @@ use std::fmt::{self, Display, Formatter};
 ///
 /// All jump tables use 0-based indexing and are expected to be densely populated. They don't need
 /// to be completely populated, though. Individual entries can be missing.
+#[derive(Clone)]
 pub struct JumpTableData {
     // Table entries, using NO_EBB as a placeholder for missing entries.
     table: Vec<Ebb>,