feat: Add Vec exercises

exercises/collections/README.md

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+### Collections
+
+Rust’s standard library includes a number of very useful data
+structures called collections. Most other data types represent one
+specific value, but collections can contain multiple values. Unlike
+the built-in array and tuple types, the data these collections point
+to is stored on the heap, which means the amount of data does not need
+to be known at compile time and can grow or shrink as the program
+runs.
+
+This exercise will get you familiar with two fundamental data
+structures that are used very often in Rust programs:
+
+* A *vector* allows you to store a variable number of values next to
+  each other.
+* A *hash map* allows you to associate a value with a particular key.
+  You may also know this by the names *map* in C++, *dictionary* in
+  Python or an *associative array* in other languages.
+
+[Rust book chapter](https://doc.rust-lang.org/stable/book/ch08-01-vectors.html)