[#26] improve efficiency of Variable lookup in Indigo

Description

To take full advantage of instructions that "jump" in the stack this was implemented:

• ability to find the (Peano) depth of a Var in the stack
• a new replaceN macro that replaces the nth element in the stack with the topmost one
• a new updateN macro that replaces the nth element in the stack with the result of an instruction (applied to the topmost element and the nth element itself)
• Data.Constraint-based functions that provide proof for the two new macros and duupX
• replacements for lookupVar that can get/set/update using the 3 macros mentioned above

Related issue(s)

Resolves #26 (closed)

Related changes (conditional)

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