Implement String Methods

[siromivel]

[okwme]

https://github.com/Arachnid/solidity-stringutils

[siromivel]

Thanks for posting this! We'll definitely check it out when we start implementing these - the focus of this issue is to create versions of the extant uint array methods for string arrays. Given Solidity's type system there's not a great way to share methods between array types (and it would possible be quite unsafe if we could figure that out,) so the plan is to mirror them for a few different types where this library will (hopefully!) be most useful. It will definitely be nice to have some safe methods for working with strings directly as we'd like to include some string[] specific methods along the lines of Array.join in other languages, of course concatenating strings in Solidity is itself non-trivial 😅