Feature request: add Number::from_i64 and Number::from_u64

[olebedev]

The use case

Currently it's not possible to construct a Value from neither i64 nor u64 but only from f64 which is a bit of a limitation.

For example, the code below will fail:

assert_eq!(
    json!(1),
    Value::Number(Number::from_f64(1)),
)

because of 1_u64 and 1_f64 mismatch. There is no other way to construct number either, only from f64.

Proposal

Add Number::from_i64 and Number::from_u64 to make it possible to construct Value::Number from these scalar types.

I would love to hear @dtolnay thoughts on the proposed addition :)

Thanks!
Oleg