Built-in vs. user-defined functions

[stevengj]

@Tamas_Papp

while all the machinery that Base & the standard libraries use to speed things up is available to the user, in a lot of cases it would just complicate the picture in a pedagogical context, especially in an intro course.

(In the case of sum, all you need is @simd to match the Base.sum performance. But even the @simd decoration is a needless complication in an introductory course.)