common/hexutil: implement TextMarshaler, TextUnmarshaler

This commit makes the wrapper types more generally applicable.
encoding.TextMarshaler is supported by most codec implementations (e.g.
for yaml).

The tests now ensure that package json actually recognizes the custom
marshaler implementation irrespective of how it is implemented.

The Uint type has new tests, too. These are tricky because uint size
depends on the CPU word size. Turns out that there was one incorrect
case where decoding returned ErrUint64Range instead of ErrUintRange.

common/hexutil/hexutil_test.go

@@ -28,9 +28,10 @@ type marshalTest struct {
 }
 
 type unmarshalTest struct {
-	input   string
-	want    interface{}
-	wantErr error
+	input        string
+	want         interface{}
+	wantErr      error // if set, decoding must fail on any platform
+	wantErr32bit error // if set, decoding must fail on 32bit platforms (used for Uint tests)
 }
 
 var (
@@ -55,6 +56,13 @@ var (
 		{uint64(0x1122334455667788), "0x1122334455667788"},
 	}
 
+	encodeUintTests = []marshalTest{
+		{uint(0), "0x0"},
+		{uint(1), "0x1"},
+		{uint(0xff), "0xff"},
+		{uint(0x11223344), "0x11223344"},
+	}
+
 	decodeBytesTests = []unmarshalTest{
 		// invalid
 		{input: ``, wantErr: ErrEmptyString},