Complete migration from "external matchfinder" to "sequence producer"

[embg]

#3484 migrates all usages in zstd.h and contrib/. However, there are some holdovers in lib/ and tests/. We should get these fixed before 1.5.5.

Tagged as release-blocking for 1.5.5, but this does not block 1.5.4.

[dahuigeniu]

good