Introduce map reduce and apply in the collections section.

[practicalli-johnny]

Demonstrate how to use map, reduce and apply to transform data structures.

Mention that in Clojure it is common to transform a data structure to make an algorithm easier, rather than to leave the data as it is and make the algorithm harder.

For example, have some map, filter and reduce exercise for Vectors
https://clojurebridgelondon.github.io/workshop/collections/vectors.html

and for maps
https://clojurebridgelondon.github.io/workshop/collections/maps.html

Suggest adding new sections (SUMMARY.md) and pages to those sections of the workshop.

To run locally, install node.js. Then install GitBook.io

npm install gitbook-cli -g

Fork ClojureBridgeLondon/workshop-content-gitbook project and clone your fork to your computer.
Change into the workshop-content-gitbook folder and load in the npm modules required to build the book locally

cd workshop-content-gitbook
gitbook install

To build the book and have live reloading of content, run

gitbook serve

Push any changes as individual PR's. Try to keep changes to one page or one section and any relevant part of the SUMMARY.md file.

Smaller PRs with a single focus are easier to review and merge. Thank you.