Add a scanner for parenthesized list

We will regularly have to parse parenthesized lists and other types of
data. Add a string scanner class that can scan for these.

Only a very simplified type of list is supported by the scanner. The
idea is that the class will be amended in order to support additional
types and more complex lists as the need arise.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>