fix(all): introduce and use iox.CopyContext (#380)

* fix(all): introduce and use iox.CopyContext

This PR is part of https://github.com/ooni/probe/issues/1417.

In https://github.com/ooni/probe-cli/pull/379 we introduced a context
aware wrapper for io.ReadAll (formerly ioutil.ReadAll).

Here we introduce a context aware wrapper for io.Copy.

* fix(humanize): more significant digits

* fix: rename humanize files to follow the common pattern

* fix aligment

* fix test

internal/engine/experiment/ndt7/ndt7_test.go

@@ -17,7 +17,7 @@ func TestNewExperimentMeasurer(t *testing.T) {
 	if measurer.ExperimentName() != "ndt" {
 		t.Fatal("unexpected name")
 	}
-	if measurer.ExperimentVersion() != "0.8.0" {
+	if measurer.ExperimentVersion() != "0.9.0" {
 		t.Fatal("unexpected version")
 	}
 }