Feature request: a unified guvectorize type string

[mcg1969]

A non-trivial fraction of my compile-time debug cycle consists of fixing unintentional inconsistencies between the JIT string and the GUFunc string in a @guvectorize decorator. I concede that I might be an odd duck in the degree to which I use generalized ufuncs, but I'd still like to see a way to unify the two strings together.

I've mocked up a way to do so here: https://gist.github.com/mcg1969/694204e08df3a7f9e327e4791af7a721

I attempted to implement this using as much of the existing approaches and code as I could, to ensure it is both as flexible and back-compatible as possible. I'm quite pleased with the results and frankly am considering using this in my own projects. I'd love to see it supported alongside the traditional two-string approach.

I'm happy to submit a PR that implements a back-compatible approach.