[#173] Generalize structure-manipulation indigo expressions

Description

Instead of having the current structure-specific operator/expression and Expr constructor, we now have:

• one generic Expr constructor that compiles to Lorentz's update
• one generic "updating" operator that simply uses that constructor
• one generic "inserting" operator based on that constructor (or even better on the previous operator)
• one generic "deletion" operator too, like the one above
• one generic "empty" instruction
• one generic "lookup" instruction

Related issue(s)

Resolves #173 (closed)

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