go back to sequence being its own class

go back to sequence being its own class
So my first attempt at getting the sequence tree hierarchy working
ended up imploding in too much complexity. So I decided to try again
in smaller steps. This version just switches back to sequence not being a
string.

one other problem is that iterators didn't seem to be serializable.
so there might be some problem writing a sequence with a tree
of annotated subsequences attached to it out.