feat: move vec exercises into their own folder

exercises/vecs/README.md

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+# Vectors
+
+Vectors are one of the most-used Rust data structures. In other programming
+languages, they'd simply be called Arrays, but since Rust operates on a
+bit of a lower level, an array in Rust is stored on the stack (meaning it
+can't grow or shrink, and the size needs to be known at compile time),
+and a Vector is stored in the heap (where these restrictions do not apply).
+
+Vectors are a bit of a later chapter in the book, but we think that they're
+useful enough to talk about them a bit earlier. We shall be talking about
+the other useful data structure, hash maps, later.
+
+## Further information
+
+- [Storing Lists of Values with Vectors](https://doc.rust-lang.org/stable/book/ch08-01-vectors.html)