Declension class fi:solakka

For the record, the historical background is that coda *j used to trigger gradation of geminate stops in some Finnish dialects (IIRC there's no evidence for gradation of singletons), but it's been mostly analogized away from existence; in particular since plural & past tense *-j has often contracted with the preceding vowel to -i-. (This has included even *oi-stem nouns such as > dial. kukoi for kukko.) However, -kka always takes the plural stem -kkoi-, so gradation to -koi- had pretty good odds for remaining.