add String#tr

somehow slower than ruby
```
./bin/crystal -e 't = Time.now; s = 0; 1000000.times{ s += "blammmmmm".tr("a", "h")[-1] }; p Time.now - t; p s' --release
2.29732
109000000
```
```
ruby -e 't = Time.now; s = 0; 1000000.times{ s += "blammmmmm".tr("a", "h")[-1].ord }; p Time.now - t; p s'
1.194307219
109000000
```