TextReader does not error immediately when encountering a null type annotation

[byronlin13]

	r := NewReaderString(`null.string ::1`)
	r.Next()
	fmt.Println(r.Err())

Currently this code snippet does not return an error. This is because the text reader parses null as a symbol token and sets the current type as string. When you call r.Next() again it will parse :: as a invalid symbol and will error out.

We should read the null and check if it is an annotation by peeking forward in the stream for ::, skipping whitespaces(tabs, new lines, etc)

[R-maan]

A partial fix in these 2 commits: 98cf37c and aef5860

They are not merged as this comment explains: #158 (comment)