SwiftLint2

Reduce calls to `String.characters.count` in `LineLengthRule.validateFile(_:)`

By applying this, the duration of linting Carthage is reduced from:
```
swiftlint lint  20.07s user 1.10s system 84% cpu 25.023 total
```
to:
```
swiftlint lint  18.27s user 1.05s system 84% cpu 22.769 total
```