feat!: hex encode id.String() output (#202)

This PR contains a breaking change that implements
#201 (comment).
Relevant #200

namespace/id.go

@@ -27,12 +27,8 @@ func (nid ID) Size() IDSize {
 	return IDSize(len(nid))
 }
 
-// String stringifies the nid.
+// String returns the hexadecimal encoding of the nid. The output of
+// nid.String() is not equivalent to string(nid).
 func (nid ID) String() string {
-	return string(nid)
-}
-
-// HexString returns hexadecimal encoding of nid.
-func (nid ID) HexString() string {
 	return hex.EncodeToString(nid)
 }

namespace/id_test.go

@@ -3,10 +3,41 @@ package namespace
 import (
 	"testing"
 
-	"github.com/stretchr/testify/require"
+	"github.com/stretchr/testify/assert"
 )
 
-func TestID_HexString(t *testing.T) {
-	nID := ID("12345678")
-	require.Equal(t, "3132333435363738", nID.HexString())
+// TestString verifies that id.String() returns the hexadecimal encoding of id.
+func TestString(t *testing.T) {
+	type testCase struct {
+		id   ID
+		want string
+	}
+	testCases := []testCase{
+		{ID(""), ""},
+		{ID("12345678"), "3132333435363738"},
+		{[]byte{0, 0, 0, 0, 0, 0, 0, 0}, "0000000000000000"},
+		{[]byte{0xaa, 0xaa, 0xaa, 0xaa, 0xaa, 0xaa, 0xaa, 0xaa}, "aaaaaaaaaaaaaaaa"},
+		{[]byte{1, 2, 3, 4, 5, 6, 7, 8}, "0102030405060708"},
+	}
+	for _, tc := range testCases {
+		assert.Equal(t, tc.want, tc.id.String())
+	}
+}
+
+// Test_string verifies that string(id) returns the native string representation of id.
+func Test_string(t *testing.T) {
+	type testCase struct {
+		id   ID
+		want string
+	}
+	testCases := []testCase{
+		{ID(""), ""},
+		{ID("12345678"), "12345678"},
+		{[]byte{0, 0, 0, 0, 0, 0, 0, 0}, "\x00\x00\x00\x00\x00\x00\x00\x00"},
+		{[]byte{0xaa, 0xaa, 0xaa, 0xaa, 0xaa, 0xaa, 0xaa, 0xaa}, "\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa"},
+		{[]byte{1, 2, 3, 4, 5, 6, 7, 8}, "\x01\x02\x03\x04\x05\x06\a\b"},
+	}
+	for _, tc := range testCases {
+		assert.Equal(t, tc.want, string(tc.id))
+	}
 }