Reorganized constraints.
Renamed Desugar to SyntaxDesugar
Renamed Syntax to SyntaxSugared.
Renamed Core to Syntax.
Moved Scheme.skeletons to ConstraintsTy.skeletons
We do not substitute constraints when finalizing.
Added a special substitution used for skeleton expansion.
Replaced substitutions with lists.
Removed unnecessary substitution
Merge branch 'feature/constraints-cleanup'
Replaced autotools with OASIS.