Arrayspec and other fixes

[panagosg7]

• Addressing issue #40 :
  • Reworked many parts of subtyping module
  • adding all visible names in the binding environment of constraints
• Sound initialization of objects using class constructors
• Adding a unique ID to each node of the AST - formerly nodes were distinguished by source span, which caused clashes.
• Replaced several occasions of syb's everywhere with visitor passes
• conflateSubElt only done after parsing
• Adding return statement to ALL void returning functions

[ranjitjhala]

Awesome thanks !