Refactor some variables and Add Heap hierarchy

[julianayang777]

• Refactor Term to Expr
• Refactor vt and bt to value and block respectively in heap modules
• Implements a lazy heap Heap_hierarchy that receives another heap h.
  • The difference between this lazy heap and the heap h is that when doing branching, the lazy heap creates a new empty heap h so that it does not need to copy the whole memory.
  • I added a new mode sh that uses this lazy heap with the write list model (sopl).

[filipeom]

Did a quick scan of the code and it looks good to me :)