Further reorganize typeclass implicits for faster implicit search

  - specific implicit for `Seq[T]`
  - make the implictits in the generated code by in a subclass
    of the low priority, expensive implicits.

After this change, we get to 187 discarded "expensive" implicits

    % scalac-hash v2.10.3 -Xlog-implicits  @args2.txt -Xprint:typer LME.scala 2>&1 | grep "not a valid" | wc -l

Down from 2200!

The benchmark compiles in 740ms, a 3.5x speedup from the original
2600ms.