Move the 'meta' dir to 'lib/cretonne/meta'.

The 'lib/cretonne' directory will be the new root of a stand-alone
cretonne crate containg both Python and Rust sources.

This is in preparation for publishing crates on crates.io.

lib/cretonne/meta/isa/__init__.py

@@ -0,0 +1,17 @@
+"""
+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 . import riscv
+
+
+def all_isas():
+    """
+    Get a list of all the supported target ISAs. Each target ISA is represented
+    as a :py:class:`cretonne.TargetISA` instance.
+    """
+    return [riscv.isa]