start adding Types for STL utilities

stl/types.py

@@ -0,0 +1,12 @@
+import typing
+
+import stl.ast as ast
+
+ML = typing.Union[ast.AtomicPred, ast.NaryOpSTL, ast.Neg]
+SL = typing.Union[ast.LinEq, ML]
+
+STL = typing.Union[SL, ast.ModalOp]
+MTL = typing.Union[ML, ast.ModalOp]
+
+PSTL = typing.NewType("PSTL", STL)
+STL_Generator = typing.Generator[STL, None, STL]