str.rs: Improved readablity by exploiting if expressions

After reading http://www.wabbo.org/blog/2014/22aug_on_bananas.html I thought the readability of this code could benefit from some if expressions and mulitple assignment.

Warning: Since multiple assignment after declaration is not supported (yet?), there is a small workaround build in, which does harm readability and performance (so i was told).