Move entity into a separate crate (#297)

lib/cretonne/Cargo.toml

@@ -14,6 +14,7 @@ build = "build.rs"
 name = "cretonne"
 
 [dependencies]
+cretonne-entity = { path = "../entity" }
 # It is a goal of the cretonne 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