Use a set instead of a map to record methods in class 'MethodGroup'

I notice there is a field `Map<MethodInvocation, MethodInvocation> methods` in the class `org.openjdk.jmh.generators.core.MethodGroup`. It is used to record all the methods of one group and the key and value of one entry of the map is always the same. From the program's meaning, I think it is good to use a set instead of a map. And this implementation confused the newbie, like me, so that I needed more time to read the related code to confirm its meaning.

So I propose to change it from a map to a set. I know the `TreeSet` is implemented by using a `TreeMap` internally, but it is clearer about the program's meaning by using a set.