Add GeneratorVariableLengthChecker

This checker restricts the length of variables's name in generator.
For example, if the max length of name is 1, checker allows

    for(i <- 0 to 10) {
      println(i)
    }

    for((k, l) <- (0 until 10, 10 until 20)) {
      println(k * l)
    }

but does not allows

    for(foo <- 0 to 10) {
      println(i)
    }

    for((foo, bar) <- (0 until 10, 10 until 20)) {
      println(foo * bar)
    }

One can set the length by modifying a config file.
This encourage to use short names in small scopes.