[dispatcher] Use more efficient iteration in matching-primary-pairs

[alexander-yakushev]

Same reasoning as #152 – non-cached dispatcher still matters; and a reduce-kv is more optimized for iterating maps than a generic for. I'm deliberately not using transients to accumulate because I assume that absolute most matching method lists will be tiny (a few elements), and the overhead of transients outweighs regular vectors on such small inputs.