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

[Guoxiong Li]

Hi all,

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.

What do you think about it? Any ideas are appreciated.

Best Regards,
-- Guoxiong
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