Create a new header src/mongo/db/exec/expression/evaluate.h and define a namespace mongo::exec::expression with a set of functions

Value evaluate(const ExpressionArray& expr, const Document& root, Variables* variables)

one for each

• ExpressionArray
• ExpressionArrayElemAt
• ExpressionFirst
• ExpressionLast
• ExpressionObjectToArray
• ExpressionArrayToObject
• ExpressionConcatArrays
• ExpressionIndexOfArray
• ExpressionIsArray
• ExpressionReverseArray
• ExpressionSortArray
• ExpressionSetDifference
• ExpressionSetEquals
• ExpressionSetIntersection
• ExpressionSetIsSubset
• ExpressionSetUnion
• ExpressionSlice
• ExpressionSize
• ExpressionZip

The body of the functions will be in a src/mongo/db/exec/expression/evaluate_array.cpp and will be taken from the evaluate() method, and the evaluate() method will be changed so that it calls the new method.