Implement Sha256Digest for Vec<u8> and &Vec<u8>and &String directly

[Julian-Wollersberger]

Previously, this didn't compile:

let input: Vec<u8> = b"some owned bytes".to_vec();
sha256::digest(input);

and the error message was pretty unhelpful:

error[E0277]: the trait bound `Vec<u8>: Sha256Digest` is not satisfied
  --> examples/knampf.rs:6:20
   |
6  |     sha256::digest(input);
   |     -------------- ^^^^^ the trait `Sha256Digest` is not implemented for `Vec<u8>`
   |     |
   |     required by a bound introduced by this call
   |
   = help: the following other types implement trait `Sha256Digest`:
             &[u8; N]
             &[u8]
             &str
             String
note: required by a bound in `sha256::digest`
  --> /home/level3/Documents/Rust/sha256-rs/src/lib.rs:48:18
   |
48 | pub fn digest<D: Sha256Digest>(input: D) -> String {
   |                  ^^^^^^^^^^^^ required by this bound in `digest`

The solution is to do a reborrow: sha256::digest(&*input);
But getting to that took a while and me and my colleges didn't understand what was happening. Apparently the Deref mechanism that lets you usually use &Vec<u8> in place of &[u8] doesn't work with the Sha256Digest trait.

So in this PR, I implement Sha256Digest for Vec<u8> and &Vec<u8> directly. This makes the first example just work.

The situation is similar with &String, which usually works the same as &str, but not here. So I added an impl for that too.

[baoyachi]

Thx @Julian-Wollersberger

[baoyachi]

New version publised,please use :

sha256 = "1.1.5"