Support for returning parsed data from `InputValidator`

I propose that `Input` and `InputValidator` should be changed to:

```rust
pub struct Input<'a, T = String> {
...
pub validation: Box>,
...
}

impl<'a, T> Input<'a, T> {
pub fn run(mut self) -> io::Result;
}

pub trait InputValidator {
type Output;

fn check(&self, input: &str) -> Result;
}
```

Currently you might need to both parse the provided string in the validator and after `Input::run` returns, which this could prevent.

Example with current `demand`:
```rust
use demand::{Input, InputValidator};

struct Parser;

fn parse(s: &str) -> Result {
let value: serde_json::Value = serde_json::from_str(s).map_err(|e| e.to_string())?;
Ok(value)
}

impl InputValidator for Parser {
fn check(&self, input: &str) -> Result<(), String> {
parse(input).map(|_| ())
}
}

fn main() -> Result<(), Box> {
let json = Input::new("Enter JSON:").validator(Parser).run()?;

let value = parse(&json).unwrap();
println!("{value:#?}");

Ok(())
}
```

With new proposed `demand`:
```rust
use demand::{Input, InputValidator};

struct Parser;

impl InputValidator for Parser {
type Output = serde_json::Value;

fn check(&self, input: &str) -> Result {
serde_json::from_str(input).map_err(|e| e.to_string())
}
}

fn main() -> Result<(), Box> {
let json = Input::new("Enter JSON:").validator(Parser).run()?;

println!("{value:#?}");

Ok(())
}
```