Split out instruction formats.

- cdsl.formats provides classes for describing instruction formats.
- base.formats provides concrete instruction format definitions.

lib/cretonne/meta/base/formats.py

@@ -6,8 +6,8 @@ Rust representation of cretonne IL, so all instruction formats must be defined
 in this module.
 """
 from __future__ import absolute_import
+from cdsl.formats import InstructionFormat
 from cdsl.operands import VALUE, VARIABLE_ARGS
-from cretonne import InstructionFormat
 from .immediates import imm64, uimm8, ieee32, ieee64, immvector, intcc, floatcc
 from .entities import ebb, sig_ref, func_ref, jump_table