Add GeneratorVariableLengthChecker

#### Add checker to restrict the length of variables in generator

This checker restricts the length of variables in generator.
For example, if the max length of the variables is 1, the checker allows

    for(i <- 0 to 10) {
      println(i)
    }

    for((k, l) <- (0 until 10, 10 until 20)) {
      println(k * l)
    }

but rejects

    for(foo <- 0 to 10) {
      println(foo)
    }

    for((foo, bar) <- (0 until 10, 10 until 20)) {
      println(foo * bar)
    }

The length is modifiable in a configuration file.
This checker assists you to use short names in small scopes.