Adds a new function TokenizeString. To make things simple to implement across backends, it takes similar parameters to Predict and pulls the string to tokenize from the Prompt parameter.
This will initially be used for returning the TokenUsage of the prompts, but I'm hopeful it'll find other uses as well.
Please review the CGo! The way that I'm boxing ints between 32 and 64 bits seems... wrong.
Minor cleanup to makefiles and gitignore
Minor test refactoring to allow for a new TokenizeString test case.