Kill the Semigroup instances with fire

[ezyang]

They don't make any sense at all, and when on-boarding @Ericson2314 and @abooij , both wondered what the right way to implement the semigroup instance was (there isn't a right way.) Can we get rid of it? I guess there is some code that uses it to merge sections together (conditionals maybe)? Can we find another way of managing this behavior?

[23Skidoo]

I guess you mean Semigroup/Monoid instances for Library/Executable and related types, and not, for example, flags. I actually tried doing that in an attempt to fix #3313, but at that point there was some (hairy) code that still used them.

[abooij]

For me, yeah, I found the Semigroup instances for build components and their fields confusing and inappropriate. What should the semigroup action on two Versions be? (luckily this instance does not exist, but the abundance of other instances makes it seem like this one is missing)

[23Skidoo]

I'm +1 on removing Semigroup instances for Library/Executable et al., in case it's not clear.