Fix Distinct with custom comparer (hash bug)

[StefanBertels]

There is a bug in .Distinct() when a custom comparer is used. PR includes test and fix.

Bug is that default GetHashCode is used -- this is not valid with a custom (key) comparer.

First both commits only fix one occurence of the bug (there are more). Some things come to my mind (before adding more tests/fixes):

1. IMHO there should be a Distinct<EQ, T, K>(...) variant which I would prefer for my use case (avoiding runtime comparer).
2. Maybe avoid 'func parameter' style Func<string,string,bool> for helper functions like this (comparer). This would avoid hiding a gethashcode "problem" (either needs to be supplied or otherwise risks performance penalty). Should be enough to allow a variant with IEqualityComparer parameter and make something like EqCompare<T> public to allow on-the-fly creation of Comparers using equals+gethashcode.
3. Do you mind merging Eq.ToEqualityComparer (Ord.ToComparer) #841 (which is related here / giving wrapper EQ => IEqualityComparer)? This would be a (semi) replacement for (1.)

PS: In case you like to stay with 'comparer func parameter': Maybe code (and bugfix) should be improved by moving IfNone and Match outside to avoid calling this within each call of of the comparer. Maybe comparer the should be non-optional (adding another signature without comparer) to clean up code.

[louthy]

I think this would probably be clearer an the intent:

    public static Seq<T> Distinct<T, K>(this Seq<T> list, Func<T, K> keySelector, Option<Func<K, K, bool>> compare = default) =>
        toSeq(Enumerable.Distinct(list, 
            new EqCompare<T>(
                (a, b) => compare.IfNone(default(EqDefault<K>).Equals)(keySelector(a), keySelector(b)), 
                a => compare.Match(Some: _  => 0, None: () => default(EqDefault<K>).GetHashCode(keySelector(a))))));

[louthy]

Thanks @StefanBertels 👍