Correctly encode strings with multi-byte UTF8 characters

All C driver methods that take in strings also take in an Int32 corresponding to the string's length.
In all these cases, we use the String.count property to get the length.
However, to handle multi-byte UTF8 characters, we should actually be using String.utf8.count.

Related reading:
https://developer.apple.com/documentation/swift/string
https://developer.apple.com/documentation/swift/string/utf8view