refactor bartiq to use single symbol for expressing pow method #74
Description
Copied from PsiQ/bartiq #138
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 ^.