Substring method for string types.
This crate provides a `substring` method on Rust string types. The method takes a start and end
character index and returns a string slice of the characters within that range.
The method is provided via the `Substring` trait which is implemented on the `&str` primitive.
# Usage
To use this crate, simply bring the `Substring` trait into scope and call the `substring` method on
your string types.
```rust
use substring::Substring;
assert_eq!("hello, world!".substring(7, 12), "world");
```
# Performance
As Rust strings are UTF-8, the algorithm for finding a character substring has temporal complexity
`O(n)`, where `n` is the byte length of the string. This is due to characters not being of
predictible byte lengths.