Self-recursion hack

[pitrou]

This PR revives the approach in PR #719. Self-recursion is supported, only with an explicit signature, and with the same signature in the recursive call(s).

[codecov-io]

Current coverage is 87.35%

Merging #1928 into master will decrease coverage by 0.16%

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+ Hits          46076      46171    +95   
- Misses         5663       5764   +101   
- Partials        911        921    +10   

1. 7 files (not in diff) in numba were modified. more
   • Misses -9
   • Partials +9
2. File ...ate_lower_listing.py (not in diff) was created. more

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[datnamer]

Awesome! Is this a step on the way to full higher order functions?

[seibert]

Not really. This sidesteps the need for changes to our type inference algorithm by pre-registering the signature of the recursive function (which is why we require an explicit type signature for recursion support). Higher order functions are a very different problem. :)

[datnamer]

Makes sense. Are there any plans to pursue higher order functions?

[seibert]

It's not a high priority at this point. We have a lot more basic issues to solve first.