Update memo struct to use additional structure as memo item instead of embedded structure

[dzmitryhil]

With the previous embedded struct, it was convenient if you read the transaction, but if not convenient if you create a transaction with the memo.

Previous ways of creating a memo:

type MyMemoItem struct {
	MemoType   data.VariableLength
	MemoData   data.VariableLength
	MemoFormat data.VariableLength
}

func main() {
	// example 1
	memos := make(data.Memos, 1)
	memos[0].Memo.MemoType = data.VariableLength("type")
	memos[0].Memo.MemoData = data.VariableLength("data")
	memos[0].Memo.MemoFormat = data.VariableLength("format")

	// example 2
	memos = []data.Memo{
		{
			Memo: struct {
				MemoType   data.VariableLength
				MemoData   data.VariableLength
				MemoFormat data.VariableLength
			}{
				MemoType:   data.VariableLength("type"),
				MemoData:   data.VariableLength("data"),
				MemoFormat: data.VariableLength("format"),
			},
		},
	}

	// example 3
	memos = []data.Memo{
		{
			Memo: MyMemoItem{
				MemoType:   data.VariableLength("type"),
				MemoData:   data.VariableLength("data"),
				MemoFormat: data.VariableLength("format"),
			},
		},
	}
}

The proposed changes are not breaking for example 1 and example 2, but breaking for example 3 which is rare in terms of usage in general. Anyway IMHO, better practice to have a common MemoItem.

memos = []data.Memo{
		{
			Memo: data.MemoItem{
				MemoType:   data.VariableLength("type"),
				MemoData:   data.VariableLength("data"),
				MemoFormat: data.VariableLength("format"),
			},
		},
	}