make Array#shift doesn't set arrays shared (bm_so_lists 4x faster), r…

…emove unnecessary nil fillings when they would be overwritten immediately (or an array doesn't escape). Eliminate lots of length checking on array construction paths. Other minor optimizations. Array#+ 2-3x faster, others depending on nil filling 5-10% faster.

git-svn-id: http://svn.codehaus.org/jruby/trunk/jruby@7998 961051c9-f516-0410-bf72-c9f7e237a7b7