convert _sentence to Scala

context: I'm working up to a fix for
https://github.com/NetLogo/NetLogo/issues/407 and I want any fix to be
easily mergeable from 5.0.x to master and headless
stop custom-generating _sentence primitive

context: I'm working up to a fix for
https://github.com/NetLogo/NetLogo/issues/407 .  sentence is currently
custom-generated, because the bytecode generator doesn't understand
variadic inputs. the custom generation makes attempts to change the
implementation far more difficult. so I think the performance value of
the custom-generation is less than the maintenance cost at this point.
improve performance of `sentence` on one long and one short input

fixes #407
Merge pull request #408 from NetLogo/issue-407

Issue 407 (slow `sentence`) fix