seq: Revised basic description to fit first part of from_loop doc,
which seems clearer.  Also added note that you can create an infinite
list by passing a function that constantly returns true.

from_loop: Gave additional detail about what the next function
is supposed to do, and noted that its result pair might contain
different types.  Added a simple example where the types are
the same.  (This example could be done more simpy using seq.
I still don't understand the purpose of from_loop.)

I moved seq before from_loop because it's a more basic version of this
kind of operation.  from_loop is more specialized, and complicated,
but its name might suggest that it's the normal way of making a lazy
list by repeated application of a function.  Users should see seq
first before trying to work out what from_loop is for.

…cluded

…cluded

…uded into add_lazylist_equal

…cluded into add_lazylist_equal