Fix for #141 (#142)

* Add Atom and Literal base classes to CDSL Ast. Change substitution() and copy() on Def/Apply/Rtl to support substituting Var->Union[Var, Literal]. Check in Apply() constructor kinds of passed in Literals respect instruction signature

* Change verify_semantics to check all possible instantiations of enumerated immediates (needed to descrive icmp). Add all bitvector comparison primitives and bvite; Change set_semantics to optionally accept XForms; Add semantics for icmp; Fix typing errors in semantics/{smtlib, elaborate, __init__}.py after the change of VarMap->VarAtomMap

* Forgot macros.py

* Nit obscured by testing with mypy enabled present.

* Typo

lib/cretonne/meta/cdsl/operands.py

@@ -5,10 +5,10 @@ from .types import ValueType
 from .typevar import TypeVar
 
 try:
-    from typing import Union, Dict, TYPE_CHECKING  # noqa
+    from typing import Union, Dict, TYPE_CHECKING, Iterable  # noqa
     OperandSpec = Union['OperandKind', ValueType, TypeVar]
     if TYPE_CHECKING:
-        from .ast import Enumerator, ConstantInt  # noqa
+        from .ast import Enumerator, ConstantInt, Literal  # noqa
 except ImportError:
     pass
 
@@ -128,6 +128,17 @@ class ImmediateKind(OperandKind):
         """
         return '{}::{}'.format(self.rust_type, self.values[value])
 
+    def is_enumerable(self):
+        # type: () -> bool
+        return self.values is not None
+
+    def possible_values(self):
+        # type: () -> Iterable[Literal]
+        from cdsl.ast import Enumerator # noqa
+        assert self.is_enumerable()
+        for v in self.values.keys():
+            yield Enumerator(self, v)
+
 
 # Instances of entity reference operand types are provided in the
 # `cretonne.entities` module.