[#470] Add `GET n` instruction

Description

This MR introduces the new GET n instruction.

GET ix gets the node at index ix of a right-combed pair.

Nodes are 0-indexed, and are numbered in a breadth-first, left-to-right fashion.

For example, a pair with 3 elements @pair a b c@ will be represented as a tree with 5 nodes:

   pair
   /   \
 a     pair
       /   \
     b       c

Where the nodes are numbered as follows:

     0
   /   \
 1       2
       /   \
     3       4

Related issue(s)

Resolves part of #470 (closed)

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