WeakEvent is no longer a FAT build only feature
EqualityComparerHelper no longer FAT build only

It makes no sense to keep EqualityComparerHelper for FAT builds only
when EqualityComparerHelper<T> has already been open to everybody.

[Similar logic applies to WeakEvent when you already have
WeakDelegateSet]
CustomEqualityComparer lo longer FAT build only

Because... I forgot to do it in the last commit.
Updated Readme

... apparently FAT features are less interesting these days ...