Harden the push_varsize instruction

1. Assert heap validity
2. Refactor the instruction for clarity
Remove redundant using and propery naming cleanup
Delete redundnant variables
Remove redundant qualifiers
More cleanup to improve readbility
Remove StackFrame.GetAt()
Add RuntimeMemory.GetStackFrames()
Make StackFrame.SetAt() private
Remvoe GetStackFrames()
Restore two debugger related functions
Merge pull request #2552 from ke-yu/mark_and_sweep

Code cleanup for executive
Merge pull request #2548 from junmendoza/VMStability

Harden the push_varsize instruction
Merge branch 'master' into Revit2015
Merge pull request #2571 from ke-yu/Revit2015

Merge master into Revit2015