Refactor to allow more general mutations

[JamesLee-Jones]

Currently, it is assumed that a mutation applied by Dredd will change the semantics of the system being instrumented and thus the mutation must be explicitly enabled with a mutant ID. Due to future work aimed at using Dredd to add extra code coverage points (in the style of mutation testing) to be utilized by a greybox fuzzer, we want it to be possible to insert semantics preserving mutations without them needing to be enabled.

This can be done by creating another Mutation class that represents a general mutation that doesn't need to be enabled (has a different Apply function) and then creating a subclass of Mutation that encapsulates the current default behaviour.

[JamesLee-Jones]

Note: It is worth considering if we want to keep mutation ID's and still output a file detailing the mutations that have been applied when the mutations are semantics preserving and don't need to be enabled.