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 ExpressionObject& expr, const Document& root, Variables* variables)

one for each

• ExpressionObject
• ExpressionBsonSize
• ExpressionFieldPath
• ExpressionGetField
• ExpressionSetField
• ExpressionInternalFindAllValuesAtPath

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