Enum#member_name should be exposed as public API

[straight-shoota]

Enum#member_name is an internal helper method that returns the name of the enum member, or nil if it's an unnamed member.
I'd like to expose this method as part of the public API because it could be valuable for other use cases.

One example is Process::Status#to_s which relies on Enum#to_s if the status is a signal exit. Enum#to_s returns the name of the member, or the numerical representation if unnamed. A number however would be indistinguishable from the exit code of a normal exit. So for Process::Status#to_s we want to use the member name if available and otherwise use the unambiguous stringification from Enum#to_s (see #15280).

I'm sure there are other use cases as well. Additionally, this serves as a simple indication whether the enum member is named or not.