added a playground of stuff I'm learning

iterate.go:
  - learning about goroutines
  - and how to represent tuples: a map[int]int seems
    simpler than `type Tuples struct { low int, high int }`

this just creates an array of some size, breaks it up into
different chunks of roughly-equivalent size, and runs a few
goroutines against the chunks to do some marginally interesting
work on them
rename `got` and ilk to `page` and ilk
add tests for Iterate.MakeOffsets()

changed return signature from:

   Iterate.MakeOffsets(page_data, number_pages) map[int]int

to

   Iterate.MakeOffsets(page_data, number_pages) (map[int]int, error)

Casting errors and nils to strings for the `is()` function is just
pure ugly. Eventually I'll tire of rolling my own testing assertions
and use testify.go, but for now I'm still learning.
move pagination into a utility module

apparently I need to move things around on disk to accomodate having
utility modules live in utils/ - having multiple .go files in utils/
seems to be an error.
a multidimensional slice
process files among 10 goroutines

maybe this is too many, but what the hell.