Add some scaffolding for building more crates.

The src/tools directory contains the cretonne-tools crate which will build
binaries for testing cretonne.

The src/libctonfile directory contains the ctonfile library crate which
provides reading and writing of .cton files.

src/tools/main.rs

@@ -0,0 +1,5 @@
+
+extern crate cretonne;
+extern crate ctonfile;
+
+fn main() {}