sync: move pool related code to sync.pool, cleanup, add a README.md

vlib/sync/pool/pool_test.v

@@ -0,0 +1,52 @@
+import time
+import sync.pool
+
+pub struct SResult {
+	s string
+}
+
+pub struct IResult {
+	i int
+}
+
+fn worker_s(p &pool.PoolProcessor, idx int, worker_id int) &SResult {
+	item := p.get_item<string>(idx)
+	println('worker_s worker_id: $worker_id | idx: $idx | item: $item')
+	time.sleep_ms(3)
+	return &SResult{'$item $item'}
+}
+
+fn worker_i(p &pool.PoolProcessor, idx int, worker_id int) &IResult {
+	item := p.get_item<int>(idx)
+	println('worker_i worker_id: $worker_id | idx: $idx | item: $item')
+	time.sleep_ms(5)
+	return &IResult{item * 1000}
+}
+
+fn test_work_on_strings() {
+	mut pool_s := pool.new_pool_processor(
+		callback: worker_s
+		maxjobs: 8
+	)
+
+	pool_s.work_on_items(['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j'])
+	for x in pool_s.get_results<SResult>() {
+		println(x.s)
+		assert x.s.len > 1
+	}
+}
+
+fn test_work_on_ints() {
+	// NB: since maxjobs is left empty here,
+	// the pool processor will use njobs = runtime.nr_jobs so that
+	// it will work optimally without overloading the system
+	mut pool_i := pool.new_pool_processor(
+		callback: worker_i
+	)
+
+	pool_i.work_on_items([1, 2, 3, 4, 5, 6, 7, 8])
+	for x in pool_i.get_results<IResult>() {
+		println(x.i)
+		assert x.i > 100
+	}
+}