Added control flow exceptions to manage non-local returns and block restarts...

Added control flow exceptions to manage non-local returns and block restarts in this recursively written interpreter
Signed-off-by: Stefan Marr <git@stefan-marr.de>

src/som/interpreter/control_flow.py

+class RestartLoopException(BaseException):
+    pass
+
+class ReturnException(BaseException):
+    
+    _immutable_fields_ = ["_result", "_target"]
+    
+    def __init__(self, result, target):
+        self._result = result
+        self._target = target
+    
+    def get_result(self):
+        return self._result
+    
+    def has_reached_target(self, current):
+        return current is self._target