Refactoring and reducing code duplication in JsonReader

[oskaredv]

Refactored code that was duplicated in several methods into its own method. Five of those methods(peek, nextDouble, nextInt, nextLong and nextString) have high cyclomatic complexity and this will help reducing that a bit.

[Marcono1234]

There is one more pattern of this in the static block at the end of the file; do you want to replace this as well? Might be good then to make peeked() package-private to avoid a synthetic accessor method (I assume).
Maybe at that point it might also make sense to merge peeked() and doPeek() into one method?

(I am not a member of this project; the maintainers will probably give you more in-depth feedback)

[Marcono1234]

There is one more pattern of this in the static block at the end of the file; do you want to replace this as well? Might be good then to make peeked() package-private to avoid a synthetic accessor method (I assume).
Maybe at that point it might also make sense to merge peeked() and doPeek() into one method?

@oskaredv, do you want to address that as well? Or do you think that this is not worth it?