Missing seq combinators

[jmid]

Both QCheck2 and QCheck are missing seq combinators.
Seq was added in OCaml 4.08 (or current lower bound), so that shouldn't be an issue.

For Gen (and arbitrary) we can piggy-back on list generation (and shrinking).

For Print we should decide on a format for printing them.
In qcheck-lin we print them as <2; 3; 22; 1; 3; 3; 3; 4; 4; 5; 8; 9; 2; 3; 3> (not a valid OCaml literal)
Nested Cons (...) will probably be too hard to read as console output for anything longer than ~3 elements...