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/humanize/humanize.go

@@ -0,0 +1,29 @@
+// Package humanize is like dustin/go-humanize.
+package humanize
+
+import "fmt"
+
+// SI is like dustin/go-humanize.SI but its implementation is
+// specially tailored for printing download speeds.
+func SI(value float64, unit string) string {
+	value, prefix := reduce(value)
+	return fmt.Sprintf("%6.2f %s%s", value, prefix, unit)
+}
+
+// reduce reduces value to a base value and a unit prefix. For
+// example, reduce(1055) returns (1.055, "k").
+func reduce(value float64) (float64, string) {
+	if value < 1e03 {
+		return value, " "
+	}
+	value /= 1e03
+	if value < 1e03 {
+		return value, "k"
+	}
+	value /= 1e03
+	if value < 1e03 {
+		return value, "M"
+	}
+	value /= 1e03
+	return value, "G"
+}