Add tcp.Listener (#15)

* begin reviewing TCP listener

* begin adding listener tests

* adding TestExchange implementation

* split out legacy and Step tests

* move listener to tcp package

* fix up tcp tests taking very long

* add xnet.TCPPool and work on TCPPool semantics

* xnet: TCPPool only accepts established connections

* xnet: getting to bottom of panic in xnet.Listener

* xnet: improve listener tests

* fix several data races in testing fixtures

* fix more synchronization things in listener test

* finalize tcplistener test

internal/ip.go

@@ -67,3 +67,23 @@ func IsZeroed[T comparable](a ...T) bool {
 	}
 	return true
 }
+
+// DeleteZeroed deletes zero values in-place contained within the
+// slice and returns the modified slice without zero values.
+// Does not modify capacity.
+func DeleteZeroed[T comparable](a []T) []T {
+	var z T
+	off := 0
+	deleted := false
+	for i := 0; i < len(a); i++ {
+		if a[i] != z {
+			if deleted {
+				a[off] = a[i]
+			}
+			off++
+		} else if !deleted {
+			deleted = true
+		}
+	}
+	return a[:off]
+}