Rename the 'cretonne' crate to 'cretonne-codegen'.

This fixes the next part of #287.

lib/codegen/Cargo.toml

@@ -0,0 +1,22 @@
+[package]
+authors = ["The Cretonne Project Developers"]
+name = "cretonne-codegen"
+version = "0.4.4"
+description = "Low-level code generator library"
+license = "Apache-2.0"
+documentation = "https://cretonne.readthedocs.io/"
+repository = "https://github.com/Cretonne/cretonne"
+readme = "README.md"
+keywords = ["compile", "compiler", "jit"]
+build = "build.rs"
+
+[dependencies]
+cretonne-entity = { path = "../entity", version = "0.4.4" }
+# It is a goal of the cretonne-codegen crate to have minimal external dependencies.
+# Please don't add any unless they are essential to the task of creating binary
+# machine code. Integration tests that need external dependencies can be
+# accomodated in `tests`.
+
+[badges]
+maintenance = { status = "experimental" }
+travis-ci = { repository = "Cretonne/cretonne" }