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/libcretonne/Cargo.toml

@@ -2,6 +2,10 @@
 authors = ["The Cretonne Project Developers"]
 name = "cretonne"
 version = "0.0.0"
+description = "Low-level code generator library"
+documentation = "https://cretonne.readthedocs.org/"
+repository = "https://github.com/stoklund/cretonne"
+publish = false
 build = "build.rs"
 
 [lib]