refactored walk-time synchronization into DEG
Start adding C target with Acc model
Merge branch 'develop-cpp' into develop_c

Conflicts:
	framework/src/ppl/delite/framework/a-ops/DeliteOps.scala
Add support for C++ target (checkpoint)
Bug fixes for sync objects
Bug fix related to nested ops
Add C++ support for Ref types and some OptiLA datastructures
Minor changes and use round-robin scheduling for accelerator
Mutable input list test
Add delitec/delite script options
C++ multiloop code generation
runtime support for c++ multiloop codegen
changes for c++ multiloop
Merge branch 'develop_c' of https://github.com/stanford-ppl/Delite into develop_c
merge with develop
Fixes for communication API implementations
Fixes for the C++ datastructure and multiloop
added copy and view sync; started to factor gpu codegen
fixed c++ multiloop sync
Bug fix for multiloop codegen and transfer functions