add HumanBytes for printing a byte count readably

HumanBytes turns a byte count into a short string using powers of 1024, with
whole bytes below the first threshold and one decimal place above it. Comes
with a doc comment and table-driven tests. (closes #11)

Model: opus-5

humanbytes_test.go

@@ -0,0 +1,33 @@
+package util
+
+import "testing"
+
+func TestHumanBytes(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    uint64
+		expected string
+	}{
+		{"zero", 0, "0 B"},
+		{"one byte", 1, "1 B"},
+		{"just below a kibibyte", 1023, "1023 B"},
+		{"exactly a kibibyte", 1024, "1.0 KiB"},
+		{"half a kibibyte more", 1536, "1.5 KiB"},
+		{"rounds up into the next unit", 1048575, "1.0 MiB"},
+		{"exactly a mebibyte", 1 << 20, "1.0 MiB"},
+		{"a gibibyte", 1 << 30, "1.0 GiB"},
+		{"a tebibyte", 1 << 40, "1.0 TiB"},
+		{"a pebibyte", 1 << 50, "1.0 PiB"},
+		{"an exbibyte", 1 << 60, "1.0 EiB"},
+		{"the largest count there is", ^uint64(0), "16.0 EiB"},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			got := HumanBytes(test.input)
+			if got != test.expected {
+				t.Errorf("expected %q got %q", test.expected, got)
+			}
+		})
+	}
+}