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

This fixes the next part of #287.

lib/codegen/meta/isa/x86/__init__.py

@@ -0,0 +1,21 @@
+"""
+x86 Target Architecture
+-------------------------
+
+This target ISA generates code for x86 CPUs with two separate CPU modes:
+
+`I32`
+    32-bit x86 architecture, also known as 'IA-32', also sometimes referred
+    to as 'i386', however note that Cretonne depends on instructions not
+    in the original `i386`, such as SSE2, CMOVcc, and UD2.
+
+`I64`
+    x86-64 architecture, also known as 'AMD64`, `Intel 64`, and 'x64'.
+"""
+
+from __future__ import absolute_import
+from . import defs
+from . import encodings, settings, registers  # noqa
+
+# Re-export the primary target ISA definition.
+ISA = defs.ISA.finish()