Improve Codec Coverage

[ansermino]

There seems to be some obvious cases missing from the codec test suite. Notably the following lines are never hit, some of which may be removable:

Encoding [32]byte:

gossamer/codec/encode.go

Lines 49 to 50 in 8fefce8

	case [32]byte:
	n, err = se.encodeByteArray(v[:])

Encoding common.Hash (this may be removable, custom type should be implicitly handled)

gossamer/codec/encode.go

Lines 59 to 60 in 8fefce8

	case common.Hash:
	n, err = se.Writer.Write(v.ToBytes())

Encoding [][]byte:

gossamer/codec/encode.go

Lines 291 to 298 in 8fefce8

	case [][]byte:
	n, err = se.encodeInteger(uint(len(arr)))
	bytesEncoded += n
	for _, elem := range arr {
	n, err = se.encodeByteArray(elem)
	bytesEncoded += n
	}

Decoding [32]byte, common.Hash (common.Hash likely removable):

gossamer/codec/decode.go

Lines 65 to 68 in 8fefce8

	case common.Hash, [32]byte:
	b := make([]byte, 32)
	_, err = sd.Reader.Read(b)
	out = b

Decoding Interface whose value's kind is Slice or Array; kind is Struct:

gossamer/codec/decode.go

Lines 293 to 294 in 8fefce8

	case reflect.Slice, reflect.Array:
	return sd.DecodeArray(t)

gossamer/codec/decode.go

Lines 300 to 301 in 8fefce8

	case reflect.Struct:
	return sd.DecodeTuple(t)

Decoding Array Pointer:

gossamer/codec/decode.go

Lines 310 to 311 in 8fefce8

	case reflect.Ptr:
	v = reflect.ValueOf(t).Elem()

Decoding Tuple (not pointer):

gossamer/codec/decode.go

Lines 375 to 376 in 8fefce8

	default:
	v = reflect.ValueOf(t)

[noot]

we should probably not remove the [32]byte and common.Hash cases, I believe some common types depend on these cases. also, you would think those two cases are duplicates, since common.Hash is an alias for [32]byte, but unfortunately the type switch doesn't recognize that :/