Add num_traits Signed to prelude

[jracollins]

First of all, thanks for the awesome crate!

Currently the .signum() method cannot be used when importing prelude without adding num-traits as a specific dependency in a downstream crate and importing manually.

Other used num_traits traits are currently reexported under the prelude module so I think this tiny change to align makes sense?

Example error message when trying to use currently:

error[E0599]: no method named `signum` found for struct `rust_decimal::Decimal` in the current scope
   --> mycode/mod.rs:465:88
    |
465 |             myvalue.signum();
    |                     ^^^^^^ method not found in `rust_decimal::Decimal`
    |
   ::: /home/james/.cargo/registry/src/github.com-1ecc6299db9ec823/num-traits-0.2.14/src/sign.rs:35:8
    |
35  |     fn signum(&self) -> Self;
    |        ------ the method is available for `rust_decimal::Decimal` here
    |
    = help: items from traits can only be used if the trait is in scope
help: the following trait is implemented but not in scope; perhaps add a `use` for it:
    |
1   | use num_traits::sign::Signed;```

[paupino]

Looks good to me. Thank you for your help and support!