Update method names to be snake case instead of camel case

[jasonwalsh]

Method names such as visitPrimitive should become visit_primitive. Snake casing is considered to be more "Pythonic."

Instances of camel cased method names exist in the following modules:

• https://github.com/pennsignals/aptos/blob/master/aptos/visitors.py
• https://github.com/pennsignals/aptos/blob/master/aptos/primitives.py