Fixed Python 3.8 not generating 'CRP' mutations (fixes #55)

[f-str]

Since Python 3.8 Num, Str, Bytes, Ellipsis and NameConstant AST nodes are replaced and combined into the Constant node. (https://greentreesnakes.readthedocs.io/en/latest/nodes.html)
Hence there is the need to declare visitor methods producing the same mutations for Constant nodes. These visitors have to return Constant AST nodes in order to produce non deprecated Python 3.8 code.