Improved constant values generated code (issue #11)

[danielhenrymantilla]

Co-authored-by: @glchapman

Fixes #11

So the enum

#[derive(TryFromPrimitive)]
#[repr(u8)]
enum Example { A, B, C, D, E, F = 8, G, H, I, J, K, L, M, }

now generates:

impl ::num_enum::TryFromPrimitive for Example {
    // ...
    fn try_from_primitive(number: Self::Primitive)
     ->
         ::core::result::Result<Self,
                                ::num_enum::TryFromPrimitiveError<Self>> {
        #![allow(non_upper_case_globals)]
        const A: u8 = 0;
        const B: u8 = u8::wrapping_add(A, 1);
        const C: u8 = u8::wrapping_add(B, 1);
        const D: u8 = u8::wrapping_add(C, 1);
        const E: u8 = u8::wrapping_add(D, 1);
        const F: u8 = 8;
        const G: u8 = u8::wrapping_add(F, 1);
        const H: u8 = u8::wrapping_add(G, 1);
        const I: u8 = u8::wrapping_add(H, 1);
        const J: u8 = u8::wrapping_add(I, 1);
        const K: u8 = u8::wrapping_add(J, 1);
        const L: u8 = u8::wrapping_add(K, 1);
        const M: u8 = u8::wrapping_add(L, 1);

[illicitonion]

Works for me :) Thanks for the improvement!