[Nim] Improve base64 encoding performance
[Nim] Improve base64 encoding performance

* Fix base64 encoder usage for better performance
* Replace deprecate function repeatChar() with repeat()

kostya/benchmarks,kostya/benchmarks,kostya/benchmarks,kostya/benchmarks,kostya/benchmarks,kostya/benchmarks,kostya/benchmarks,kostya/benchmarks,kostya/benchmarks,kostya/benchmarks,kostya/benchmarks,kostya/benchmarks,kostya/benchmarks,kostya/benchmarks