Add S-expression parsing example

[MarkMcCaskey]

resolves #952

Hello!

Thanks for taking the time to review this! Let me know if there are any changes I can make to improve this example -- I'm happy to make any changes.

[MarkMcCaskey]

Great! Thank you for your feedback! I'm very surprised I had never seen map(Constructor) before -- that's very useful.

I've updated the example to reflect the changes you suggested!

[Geal]

awesome! One last thing: you should add alloc as feature to the example like this: https://github.com/Geal/nom/blob/master/Cargo.toml#L154
Otherwise the no_std CI variant won't pass: https://travis-ci.org/Geal/nom/jobs/545411894

[MarkMcCaskey]

Okay! I updated it to be like the JSON example and squashed the extra commits. Thanks for the guidance!

[MarkMcCaskey]

It looks like the reason CI is failing now is unrelated to my changes

[Geal]

thank you for your help :)