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

This fixes the next part of #287.

lib/codegen/meta/isa/__init__.py

@@ -0,0 +1,24 @@
+"""
+Cretonne target ISA definitions
+-------------------------------
+
+The :py:mod:`isa` package contains sub-packages for each target instruction set
+architecture supported by Cretonne.
+"""
+from __future__ import absolute_import
+from cdsl.isa import TargetISA  # noqa
+from . import riscv, x86, arm32, arm64
+
+try:
+    from typing import List  # noqa
+except ImportError:
+    pass
+
+
+def all_isas():
+    # type: () -> List[TargetISA]
+    """
+    Get a list of all the supported target ISAs. Each target ISA is represented
+    as a :py:class:`cretonne.TargetISA` instance.
+    """
+    return [riscv.ISA, x86.ISA, arm32.ISA, arm64.ISA]