Credence.Rule.AvoidGraphemesEnumCount (credence v0.2.0)

Performance rule: warns when String.graphemes/1 |> Enum.count() is used.

• For Enum.count/1 (no predicate): suggests String.length/1, which avoids allocating an intermediate list of graphemes.

• For Enum.count/2 (with a predicate): suggests manual binary recursion over the string, which avoids allocating the intermediate grapheme list entirely.