Add first-class and higher-order functions

glossary/First-class-function.md

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+### First-class function
+
+A programming language is said to have first-class functions if it treats them as first-class citizens, meaning they can be passed as arguments, be returned as values from other functions, be assigned to variables and stored in data structures.

glossary/Higher-order-function.md

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+### Higher-order function
+
+Higher-order functions or functors are functions that either take other functions as arguments, return a function as a result, or both.