work on global constraints
Current module scopes are now stored when visited, rather than obtained through complex logic
stores global requires clause when we encounter the module
global constraints are added as givens to each VC
Merge pull request #124 from yushan87/vcgen-globalconstraints

VCGenerator: Global Constraints
Grammar fixes to set construction
work in progress
Set Collection Exp done. Need to add its type in Populator
formatted some files
Merge pull request #125 from yushan87/set-exp

Set exp