Enum discriminants should be taken from the end of the input string, like lengths

[fitzgen]

trafficstars

The same rationale for why we take lengths from the end of the input byte string applies to discriminants: depending on the discriminant chosen, it changes which subsequent arbitrary calls are made and how many bytes are needed. It generally seems best to have "control flow"-y choices that affect shape be taken from the end of the input string and "filler" data taken from the start.

To do this we'll need to expose our internal int_in_range that takes from the end. Not sure what to call it, or how to explain why there are two variants in the documentation well...

[Manishearth]

We should do this for dataful enums, C-like enums should still just fetch a number from the cursor position.