fix: Rename merges to cluster_merges in output of cluster_leading_eigen()

[krlmlr]

Closes #797.

[szhorvat]

While function names now avoid dots, "key names" (what are the names that come after $ called in R?) still typically use them. What's conventional in R? Is cluster.merges or cluster_merges better here?

[krlmlr]

Element names? Names of a vector?

Let's stick with the existing convention, consistency is key. If written from scratch, we could use underscores, dots are fine too. Spaces or other non-syntactic characters would make things difficult.