Generic arguments

[danieleades]

fixes #7
I've made most of the functions generic over the input they actually need, rather than concrete types. I think there's still some work to be done to improve the api.

this is a minor breaking change as seen below-

before

fn example(s: String) {
   let x: String = s;
}

fn main() {
   let my_str : &'static str = "example string";

  // this won't compile after the change as type ascriptions would be required to determine how to apply 'into()'
   example(my_str.into())
}

after

fn example<S: Into<String>>(s: S) {
   let x: String = s.into();
}

fn main() {
   let my_str : &'static str = "example string";

  // consumer of library can now use any type which can be converted into a concrete string to call the function
   example(my_str)
}