Cast and string utils

[nanaknihal]

I made a few new utils for type casts and parsing strings (represented as u8 arrays). I've tested them locally but was having difficulty adding and running tests in zokrates_stdlib/tests/tests/utils. Compiler was saying were saying the new utils were not found, even though I rebuilt after adding the utils. I rebuilt with these instructions.

I also wrote a base64 conversion string util which could be helpful, but requires these as dependencies, so perhaps let's make sure I contribute these correctly before submitting those.