Use *_test packages to make examples copy-pasteable

[kke]

The main reason for this PR was to make the examples in documentation
copy-pasteable, but there are some additional benefits too.

For example, the pool example before this change:

p := New().WithMaxGoroutines(3)
for i := 0; i < 5; i++ {
	p.Go(func() {
		fmt.Println("conc")
	})
}
p.Wait()

and after:

p := pool.New().WithMaxGoroutines(3) // has package name
for i := 0; i < 5; i++ {
	p.Go(func() {
		fmt.Println("conc")
	})
}
p.Wait()

The function defaultMaxGoroutines in iter package had to be exported because it was used from the tests.

[codecov-commenter]

Codecov Report

Merging #124 (4f9a721) into main (b3c39d3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #124   +/-   ##
=======================================
  Coverage   99.31%   99.31%           
=======================================
  Files          12       12           
  Lines         441      441           
=======================================
  Hits          438      438           
  Misses          3        3           

📣 Codecov offers a browser extension for seamless coverage viewing on GitHub. Try it in Chrome or Firefox today!

[camdencheek]

Great idea! Neat trick -- that's a new one for me. Thanks!