Poc dunder without specialization

[konstin]

This is a prove of concept to how we can support dunder methods/protocols without specialization using inventory. Currently the only implemented method is __add__.

I don't have the time to bring this into a mergeable state or extend it to all dunder methods, so feel free to take this branch as a starting point.

[codecov]

Codecov Report

Merging #552 into master will decrease coverage by 0.12%.
The diff coverage is 78.26%.

@@            Coverage Diff             @@
##           master     #552      +/-   ##
==========================================
- Coverage   87.61%   87.49%   -0.13%     
==========================================
  Files          65       65              
  Lines        3449     3470      +21     
==========================================
+ Hits         3022     3036      +14     
- Misses        427      434       +7

Impacted Files	Coverage Δ
src/ffi3/object.rs	93.33% <ø> (ø)	⬆️
src/type_object.rs	89.2% <100%> (ø)	⬆️
src/class/number.rs	92.9% <71.42%> (-1.3%)	⬇️
src/class/methods.rs	86.04% <80%> (-3.24%)	⬇️

---

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[codecov]

Codecov Report

Merging #552 into master will decrease coverage by 0.12%.
The diff coverage is 78.26%.

@@            Coverage Diff             @@
##           master     #552      +/-   ##
==========================================
- Coverage   87.61%   87.49%   -0.13%     
==========================================
  Files          65       65              
  Lines        3449     3470      +21     
==========================================
+ Hits         3022     3036      +14     
- Misses        427      434       +7

Impacted Files	Coverage Δ
src/ffi3/object.rs	93.33% <ø> (ø)	⬆️
src/type_object.rs	89.2% <100%> (ø)	⬆️
src/class/number.rs	92.9% <71.42%> (-1.3%)	⬇️
src/class/methods.rs	86.04% <80%> (-3.24%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 58e17be...566a70c. Read the comment docs.

[gilescope]

Doesn't the selection of the PyNumberProtocolImpl impl depend upon specialisation? This branch doesn't compile for me on nightly 2019-11-09.