Move library crates under 'lib/'.

Give these crates each a more standard directory layout with sources in
a 'src' sub-sirectory and Cargo.toml in the top lib/foo directory.

Add license and description fields to each.

The build script for the cretonne crate now lives in
'lib/cretonne/build.rs' separating it from the normal library sources
under 'lib/cretonne/src'.

lib/cretonne/Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+authors = ["The Cretonne Project Developers"]
+name = "cretonne"
+version = "0.0.0"
+description = "Low-level code generator library"
+license = "Apache-2.0"
+documentation = "https://cretonne.readthedocs.io/"
+repository = "https://github.com/stoklund/cretonne"
+publish = false
+build = "build.rs"
+
+[lib]
+name = "cretonne"
+
+[dependencies]
+# 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
+# accomodated in `tests`.