rpc: implement JSON marshaling of BlockNumber

[zenovich]

Currently rpc.BlockNumber is marshalled to JSON as a numeric value {"blockNumber":123} which is wrong because BlockNumber.UnmarshalJSON() wants it either to be hex-encoded or to have a special value ("earliest"/"latest"/"pending"). As a result, the chain rpc.BlockNumberOrHashWithNumber(123) -> json.Marshal() -> json.Unmarshal() fails with an error "cannot unmarshal object into Go value of type string".

The PR fixes the issue by adding BlockNumber.MarshalText() method which outputs either a hex-encoded number or a special string depending on the given block number.

[holiman]

I think I agree with the argument made in this PR, not sure if there are any consequences that we need to consider...?

[holiman]

Might skip a layer of test nesting:

Suggested change

	for _, test := range tests {
	test := test
	t.Run(test.name, func(t *testing.T) {
	bnh := BlockNumberOrHashWithNumber(BlockNumber(test.number))
	marshalled, err := json.Marshal(bnh)
	if err != nil {
	t.Fatal("cannot marshal:", err)
	}
	var unmarshalled BlockNumberOrHash
	err = json.Unmarshal(marshalled, &unmarshalled)
	if err != nil {
	t.Fatal("cannot unmarshal:", err)
	}
	if !reflect.DeepEqual(bnh, unmarshalled) {
	t.Fatalf("wrong result: expected %v, got %v", bnh, unmarshalled)
	}
	})
	}
	}
	for _, test := range tests {
	bnh := BlockNumberOrHashWithNumber(BlockNumber(test.number))
	marshalled, err := json.Marshal(bnh)
	if err != nil {
	t.Fatal("cannot marshal:", err)
	}
	var unmarshalled BlockNumberOrHash
	if err = json.Unmarshal(marshalled, &unmarshalled); err != nil {
	t.Fatal("cannot unmarshal:", err)
	}
	if !reflect.DeepEqual(bnh, unmarshalled) {
	t.Fatalf("wrong result: expected %v, got %v", bnh, unmarshalled)
	}
	}
	}

[zenovich]

test.name would become unused

[zenovich]

There are two pros of test nesting here:

1. failure related to one dataset doesn't suppress checking of other datasets,
2. in case of failure, the report clearly shows for which of data sets the code doesn't work correctly.

[zenovich]

--- FAIL: TestBlockNumberOrHash_WithNumber_MarshalAndUnmarshal (0.00s)
    --- FAIL: TestBlockNumberOrHash_WithNumber_MarshalAndUnmarshal/pending (0.00s)
        types_test.go:148: cannot unmarshal: json: cannot unmarshal object into Go value of type string
    --- FAIL: TestBlockNumberOrHash_WithNumber_MarshalAndUnmarshal/latest (0.00s)
        types_test.go:148: cannot unmarshal: json: cannot unmarshal object into Go value of type string