Convert `spec_type` from `enum` to `enum class`

To improve the type-safety of our code and make it less error-prone, we should convert spec_type (located inside struct MutateInSpecs) from an enum to an enum class.

References:

1. https://agrawalsuneet.github.io/blogs/enum-vs-enum-class-in-c++/
2. https://stackoverflow.com/questions/18335861/why-is-enum-class-considered-safer-to-use-than-plain-enum