implements strcat and strrchr

[rafaelbeckel]

No description provided.

[thejpster]

Thank you for this PR! I left a few notes about the code organisation.

[rafaelbeckel]

@thejpster I addressed the comments and kept each function in their separate files.

My editor applied some minor linting rules to the README; I left them there, but I can revert if you'd like.

[thejpster]

Please do revert the README changes - it hides the actual change. Please could you also ensure your doc comments start with a single line of text and then a couple of newlines. If you look at the cargo doc before and after you'll see that this simplifies the summary for each function.