bugfix(derive) implicit discriminants were wrongly calculated

[nduchaux]

When a enum variant had no explicit discriminant, there was a try to calculate it by looking at the previous one.
This was not working at all:

• the first enum variant was never looked at
• when finding an enum variant with a discriminant, it was taking its value + 1 (even if it was not the immediate previous one).

This commit corrects the logic to make it work.

[nduchaux]

The second commit permits to "parse" any numeric literals (e.g. 0x1234). This was not the case for the first one (or even before that).
For simplifity, I had to add a dependancy to eval the result from the base discriminant with the "offset".