Support for NameConstant mutations

• Added new mutation support for NameConstant in AST.
• This includes substitutions for singleton assignments such as: True, False, and None.
• This is the first non-type mutation and required adding a readonly parameter to the transformers.MutateAST class. Additionally, the type-hints for the LocIndex and MutationOpSet were updated to Any to support the mixed types. This was more flexible than a series of overload signatures.