Fixing symbolic backend in unified-builtins2.
fixed the sort computation in KItem and AddInjection pass
disable io in prove
fixed the indexing of I/O cell that contains only a single variable
revised reverse_spec.k
included sum_iterative in the regression test
addressed Andrei's comments
fixed indexing on I/O cells to be more general
proved the comm_assoc example
Merge pull request #815 from yilongli/fixing-issue-781

fixing issue 781
Merge pull request #814 from andreiarusoaie/unified-builtins2

Fixing symbolic backend in unified-builtins2.
Merge pull request #816 from yilongli/prove-simple-iterative-sum

Prove simple iterative sum
Merge branch 'unified-builtins2' into pdmc2