adds as_number to Value

[chanced]

Resolves #1068 by adding as_number to Value.

[chanced]

/// If the `Value` is an Number, returns the associated [`Number`]. Returns None
/// otherwise.
///
/// ```
/// # use serde_json::{json, Number};
/// #
/// let v = json!({ "a": 1, "b": 2.2, "c": -3, "d": "4" });
///
/// // The number `1` is an u64.
/// assert_eq!(v["a"].as_number(), Some(&Number::from(1u64)));
///
/// // The number `2.2` is an f64.
/// assert_eq!(v["b"].as_number(), Some(&Number::from_f64(2.2).unwrap()));
///
/// // The number `-3` is an i64.
/// assert_eq!(v["c"].as_number(), Some(&Number::from(-3i64)));
///
/// // The string `"4"` is not a number.
/// assert_eq!(v["d"].as_number(), None);
///  
/// ```
pub fn as_number(&self) -> Option<&Number> {
    match self {
        Value::Number(number) => Some(number),
        _ => None,
    }
}

[dtolnay]

Thanks!

[chanced]

woops, I just realized I made 2 copypasta mistakes in the comment.

• "an Number" should be "a Number.
• "The number 1 is an u64." should be "The number 1 is a u64"

I'm sorry about not catching it sooner!