Rename meta/target -> meta/isa.

Clarify terminology by always referring to a 'Target ISA' instead of just
'Target'. Use 'isa' as a module name instead of 'target' both in Rust and Python
code.

This is only to clarify terminology and not at all because Cargo insists on
using the 'target' sub-directory for build products. Oh, no. Not at all.

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 . 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]