Fix the line counting in 'string' function.

The line counter becomes invalid, when the rules with linewrap are
used. This issue appears, because the parsing FSM does not rollback
the line counter after attempting such rule.
Add unit test for 'string' function in leex test suite.
Fix the line counting in 'token' and 'tokens' functions.

The line counter becomes invalid, when the rules with linewrap are
used. This issue appears, because the parsing FSM does not rollback
the line counter after attempting such rule.
Unit tests for 'token' and 'tokens' are also added.