Add REF nodes for lazy allocation, and extract closed sub-exprs into new definitions

[Boscop]

This PR adds the REF nodes for lazy allocation of def sub-nets.
It also implements extraction of sub-terms into new definitions when possible.
E.g.

  def double = λn (n (λp (S (S (double p)))) Z)

gets pre-processed to:

  def double = λn (n EXTRACTED_0 Z)
  def $EXTRACTED_0 = λp (S (S (double p)))

but only when definition_book.extract_closed_subterms(); is called (like it is in main.rs).
Currently, main is printing the before and after:

definition_book.print();
definition_book.extract_closed_subterms();
definition_book.print();

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When a REF node is interacted with, it gets substituted with its term's net, and then the normal rewriting continues.