refactor bartiq to use single symbol for expressing `pow` method

[mstechly]

Currently in Bartiq we allow to express pow function as both ^ and **.
This is problematic since:

• ^ in Python is a XOR operator,
• an expression including one symbol could be later serialized and the symbol might turn into a different one, which could be confusing.

After some discussion with @dexter2206, we think that using ** would be better than ^.