exporting in-csv
moved iterate clauses around, added iterate clause for sampling

The `sampling` clause uses reservoir sampling to randomly generate a
subset of the original series, returns a list.

Examples:
{{{#!lisp
(iter (for row in-csv "1,2,3
4,5,6
7,8,9")
  (sampling row size 2))
=> (("1" "2" "3") ("7" "8" "9"))
(iter (for row in-csv "1,2,3
4,5,6
7,8,9")
  (sampling row into sample size 2)
  (finally (break "got the sample ~a" sample)))
}}}

refs Programming:#310(.5)
don't need to export the iterate symbols
use iterate clauses for internal csv reading

refs Programming:#310 (.5)