string in Rust

The String object type is provided in Standard Library. Unlike string literal, the string object type is not a part of the core language. It is defined as public structure in standard library pub struct String. String is a growable collection. It is mutable and UTF-8 encoded type. The String object type can be used to represent string values that are provided at runtime. String object is allocated in the heap.
```
fn main(){
   let empty_string = String::new();
   println!("length is {}", empty_string.len());

   let string = String::from("Rust");
   println!("length is {}", string.len());
}
```
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