Fix String.concat ignoring the len parameter
Fix String.concat ignoring the len parameter

The loop pushing values from iter didn't increment n, resulting in finite iterators getting concated completely.
Also infinite iterators such as `Iter.repeat_value` would result in the function never returning and eating up memory.
CausalityLtd/ponyc,cquinn/ponyc,sgebbie/ponyc,Theodus/ponyc,sgebbie/ponyc,CausalityLtd/ponyc,cquinn/ponyc,jemc/ponyc,cquinn/ponyc,kulibali/ponyc,ponylang/ponyc,sgebbie/ponyc,Theodus/ponyc,kulibali/ponyc,dipinhora/ponyc,sgebbie/ponyc,cquinn/ponyc,jemc/ponyc,Theodus/ponyc,dipinhora/ponyc,sgebbie/ponyc,jemc/ponyc,Theodus/ponyc,kulibali/ponyc,Theodus/ponyc,kulibali/ponyc,ponylang/ponyc,dipinhora/ponyc,ponylang/ponyc,CausalityLtd/ponyc