Include mention of functional programming

[DoubleHyphen]

Tour of Rust has extensive mention of Rust's OOP capabilities, yet there is no mention of its FP capabilities. A quick overview of the TOC doesn't even appear to teach how to create closures! I believe that at the absolute minimum there should be a note about closures, map & filter and maybe also and_then, and quite possibly a small note on how to return functions from functions. I could attempt to write those myself if this is permitted.

[Zerotask]

@DoubleHyphen Yes, I agree with you. This should definitely covered.