Add `v2.SimpleCalculator` sample
Move plugin to `parboiled2` namespace
Move `OpTree` string representations to `lazy vals`
Implement first step to `parboiled1`'s `ReportingParseRunner`. I.e. find furthest matched `pos` in `input`

Implement simple debug tracing
Add `debug` variable to enable/disable debugging
Implement `ErrorUtils` -- expected `Rule` searcher and pretty priter
Implement `ErrorUtils` to be used as `Parser` wrapper
Move `calculators` examples to another namespace