Discourage misuses of `T::Enum` about rubocop-sorbet

[amomchilov]

it is also less performant than implementing the same functionality using data structures from Ruby (e.g. list of constants)

The most common operation done with enum values is to compare them (either directly with ==, or indirectly via case/when/===)

T::Enum#== and T::Enum#=== can be made just as fast as comparing string/symbol constants (in either case, it just uses the identity-based comparison inherited from Object#==).

I explore this in https://github.com/Shopify/shopify-types/pull/454/files , but had trouble getting CI to validate my changes. Perhaps that's worth exploring first?

I think that change (having #== and #=== only be overridden when T::Configuration.legacy_t_enum_migration_mode? is set) can even be upstreamed into sorbet/sorbet.

[egiurleo]

@amomchilov That's really interesting! I will take a look at this as it ties in well with our Sorbet performance project. I wonder if we could apply the same logic to the <=> method? I did some benchmarks where </> comparisons were 8x slower with T::Enum values than constants, which was pretty wild to me.

[amomchilov]

Hmm, when is <=> even used? T::Enum doesn't even include Comparable