Add a kind of hacky Frankenstein C version of doing the validation

This is based on bits of code I had lying around from other projects. It can probably be cleaned up and made much more canonical - I'm very out of practice with C so someone is likely to vomit from how bad this is (especially since it's gratuitously not C89).




There's a bug somewhere in here where token is truncated 

```bash
% ./token 
Your Token:   Zkr7TIflOYl/6wkahr+GG1u7LWZJzLl1
Fastly Token: Zkr7TIflOYl/6wkahr+GG1u7LWZJzLl1ACdaIRvK5Ik=
Validation:   Token Validation Failed!

%  ./token 
Your Token:   hNWbxPpRuJj878/aW3c/CfGpDZuKhiQzURfXq9HnfLI=
Fastly Token: hNWbxPpRuJj878/aW3c/CfGpDZuKhiQzURfXq9HnfLI=
Validation:   Token Validation Succeeded!
```