Fix u64 deserialization

[litchipi]

Past issue: toml-rs/toml-rs#256

u64 cannot be deserialized as it's backed by a i64 format

This causes this to fail:

let data = format!("id = {}", u64::MAX);
let val : toml::Value = toml::from_str(&data).unwrap();
println!("{:?}", val);

And any number > i64::MAX will also fail.

[epage]

This deals with problems similar to #540

[litchipi]

Indeed, I hacked a bit around, and it just makes sense to have a Number(i64) variant to cover the everyday usage.
We could imagine some kind of variants for larger numbers, but the toml::Value enum would be much more of a burden (many variants to take care of).

I guess the best way would be to have:

enum Value {
   // other variants ...
   Number(NumberValue),
}

enum NumberValue {
  Unsigned(u64),
  Signed(i64),
  BigUnsigned(u128),
  BigSigned(i128),
  Float(f64),
}

impl TryInto<u64> for NumberValue {
  // If the variant matches, get the value, else raise an error
}

It forces an extra step to get a usable number from the toml::Value, but it covers all kind of numeric types we could want.
Seams to me like a good middle ground