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

cmd/ooniprobe/internal/config/parser_test.go

@@ -1,12 +1,14 @@
 package config
 
 import (
+	"context"
 	"crypto/sha256"
 	"encoding/hex"
-	"io"
 	"io/ioutil"
 	"os"
 	"testing"
+
+	"github.com/ooni/probe-cli/v3/internal/iox"
 )
 
 func getShasum(path string) (string, error) {
@@ -17,7 +19,7 @@ func getShasum(path string) (string, error) {
 		return "", err
 	}
 	defer f.Close()
-	if _, err := io.Copy(hasher, f); err != nil {
+	if _, err := iox.CopyContext(context.Background(), hasher, f); err != nil {
 		return "", err
 	}
 	return hex.EncodeToString(hasher.Sum(nil)), nil