Add the very basics of Intel 32-bit instruction encodings.

Tabulate the Intel opcode representations and implement an OP() function
which computes the encoding bits.

Implement the single-byte opcode with a reg-reg ModR/M byte.

lib/cretonne/meta/isa/intel/__init__.py

@@ -17,7 +17,7 @@ is no x87 floating point support.
 
 from __future__ import absolute_import
 from . import defs
-from . import settings, registers  # noqa
+from . import encodings, settings, registers  # noqa
 
 # Re-export the primary target ISA definition.
 ISA = defs.ISA.finish()