Using list variables as scalar variables

[spooning]

Originally submitted to Google Code by @pekkaklarck on 29 Sep 2008

Sometimes it would be useful to be able to use a list variable so that it
would be considered a scalar variable containing a list. Possible use cases
include getting the length of the list variable using built-in 'Get Length'
keyword and modifying the list with different keywords from Collections
library.

A simple solution is using ${LIST} syntax for using @⁠{LIST} as a scalar. We
don't need to add any new syntax and this is also trivial to implement. All
that needs to be done is checking is a variable @⁠{XXX} set and returning
its value, if resolving ${YYY} fails. To make this change 100% backwards
compatible (and more logical in general), we should always return the value
of ${NAME} if it's set, regardless is @⁠{NAME} defined or not.

[spooning]

Originally submitted to Google Code by @pekkaklarck on 2 Oct 2008

Tests committed in r858 and implementation in r859. Must be documented in the user
guide still.

[spooning]

Originally submitted to Google Code by @pekkaklarck on 2 Oct 2008

Need to mention this also in the doc of Collections library.

[spooning]

Originally submitted to Google Code by @pekkaklarck on 9 Oct 2008

User guide changes committed in r919 and Collections changes in r923.