Add a functional programming example

[twalmsley]

This example is an attempt at writing code in a style that results in a more readable program flow. The test method clearly shows the steps in the use case, even if there looks to be a lot of code. The hope is that the fine-grained functions can be made reusable in other contexts so that composing new use cases from existing functions may become easier.

[twalmsley]

The real question for this PR is whether the code is any easier to read and understand, and is it better structured - if not then I don't mind if this PR is rejected.

[GCHQDeveloper42]

I would prefer this to be made an executable class like the other examples, rather than a unit test.

[twalmsley]

I would prefer this to be made an executable class like the other examples, rather than a unit test.

I can move it to src/main/java and add main() but will it ever be executed? We can't be sure the examples are correct unless they're executed when code changes - we might find out months later. It could be added as a build step but would need updating for every new example.

[GCHQDeveloper42]

I can move it to src/main/java and add main() but will it ever be executed? We can't be sure the examples are correct unless they're executed when code changes - we might find out months later. It could be added as a build step but would need updating for every new example.

I take your point. Could you define the program in an example class and then execute it in a FunctionalProgrammingExampleTest suite?