feat!: add `DiscriminantValue` to `Definition::Enum::variants` tuples

[dj8yfo]

resolves #230

[dj8yfo]

this commit was added to restrict possible values of discriminants to u8 range for BorshSchema to be congruent with what BorshSerialize, BorshDeserialize derives do.

---

with it:

    #[derive(BorshSchema)]
    #[borsh(use_discriminant = true)]
    enum XYa {
        A,
        B = discrim(),
        C,
        D,
        E = -100,
        F,
    }

    const fn discrim() -> isize {
        0x14
    }
//     |
// 277 |     #[derive(BorshSchema)]
//     |              ----------- expected due to this
// ...
// 281 |         B = discrim(),
//     |             ^^^^^^^^^ expected `u8`, found `isize`
//     |
// help: you can convert an `isize` to a `u8` and panic if the converted value doesn't fit
// 4  error[E0277]: the trait bound `u8: Neg` is not satisfied
//    --> borsh/tests/test_enum_discriminants.rs:284:13
//     |
// 284 |         E = -100,
//     |             ^^^^ the trait `Neg` is not implemented for `u8`
//     |
//     = help: the following other types implement trait `Neg`:

---

Without it:

    #[derive(BorshSchema)]
    #[borsh(use_discriminant = true)]
    enum XYa {
        A,
        B = discrim(),
        C,
        D,
        E = -100,
        F,
    }

    const fn discrim() -> isize {
        0x14
    }
//  1  error[E0308]: mismatched types
//     --> borsh/tests/test_enum_discriminants.rs:281:13
//      |
//  281 |         B = discrim(),
//      |             ^^^^^^^^^ expected `i64`, found `isize`
//      |
//  help: you can convert an `isize` to an `i64` and panic if the converted value doesn't fit

but

    #[derive(BorshSchema)]
    #[borsh(use_discriminant = true)]
    #[repr(i64)]
    enum XYa {
        A,
        B = discrim(),
        C,
        D,
        E = -100,
        F,
    }

    const fn discrim() -> i64 {
        0x14
    }
// OK