Implement letting

[niklasdewally]

Add letting declarations.

Instead of just storing decision variables, let the symbol table be of type:

SymbolTable<Name,RefTo>

where RefTo is an enum:

enum RefTo {
  Letting(Letting),
  DecisionVariable(DecisionVariable),
 // and so on
 }

To-do

• Make symbol table store a ReferenceTo enum, with variants for lettings and decision variables #522
• Parseletting declarations (json)
• Parseletting declarations (tree-sitter)
• Rule to substitute lettings for their values before passing to Minion.