add shortcut to serialize/deserialize based on question at FV hangout

use case
```
from z3 import *

x, y = Ints('x y')
s = (x + y).serialize()

y = deserialize(s)

print(y)
```

src/api/python/z3/z3.py

@@ -1101,6 +1101,28 @@ def children(self):
         else:
             return []
 
+    def from_string(self, s):
+        pass
+
+    def serialize(self):
+        s = Solver()
+        f = Function('F', self.sort(), BoolSort(self.ctx))
+        s.add(f(self))
+        return s.sexpr()
+
+def deserialize(st):
+    """inverse function to the serialize method on ExprRef.
+    It is made available to make it easier for users to serialize expressions back and forth between
+    strings. Solvers can be serialized using the 'sexpr()' method.
+    """
+    s = Solver()
+    s.from_string(st)
+    if len(s.assertions()) != 1:
+        raise Z3Exception("single assertion expected")
+    fml = s.assertions()[0]
+    if fml.num_args() != 1:
+        raise Z3Exception("dummy function 'F' expected")
+    return fml.arg(0)
 
 def _to_expr_ref(a, ctx):
     if isinstance(a, Pattern):