Support proposition decoding in optics

[fogfish]

type reqThread struct {
	author core.AccountID
	story  core.StoryID
	user   core.AccountID
	bias   geohash.GeoHash
}

author, account, user, bias := µ.Optics4[reqThread, core.AccountID, core.StoryID, core.AccountID, geohash.GeoHash]("author", "story", "user", "bias")

Here, the request consists of two core.AccountID. Optics would not support it without hints - "author", "story", "user", "bias".
It would be perfect if it behave like real hseq core.AccountID :: core.StoryID :: core.AccountID :: geohash.GeoHash mapped by the proposition of the type.