eval scheme: fix few crashes caused by invalid json serialization

app.py

@@ -13,6 +13,7 @@ from sensors import sensors
 import phasectrl
 from time import perf_counter, sleep
 
+from scm import stringify
 
 def create_app():
     app = Flask('hakkoso')
@@ -91,7 +92,7 @@ def handle_new_client():
 @socketio.on('eval scheme')
 def eval_scheme(code):
     output = phasectrl.safe_eval(code, statemachine.env, notify=False)
-    socketio.emit('eval output', output)
+    socketio.emit('eval output', stringify(output, quote=False))
 
 @socketio.on('get sensors history')
 def send_sensors_history(elements=1000):