Add an enable_verifier setting.

This is off by default, but enabled by the parser when reading a textual
IL file. Test files can still override the default to turn off
verification.

The setting enables IL verifier passes at critical points of the
compilation pipeline.

lib/cretonne/src/settings.rs

@@ -271,6 +271,7 @@ mod tests {
         assert_eq!(f.to_string(),
                    "[shared]\n\
                     opt_level = \"default\"\n\
+                    enable_verifier = false\n\
                     is_64bit = false\n\
                     is_compressed = false\n\
                     enable_float = true\n\