WIP: preproject and project implementation

[mzgubic]

Alternative to #380 and #382. See usage in JuliaDiff/ChainRules.jl#457. Uses @mcabbott's preproject(x) idea (somewhere I can't find) instead of a closure. This is cool because it allows the project(T, dx) to be extended depending on both the

Some observations (comments welcome):

• This PR is cool because similarly to WIP: projector implementation (returning a closure) #382 it allows us to not close over primal value (x)
• This PR is cool because unlike WIP: projector implementation (returning a closure) #382, where the returned closure can not be extended, the project function can be extended by someone developing a Quaternions package. They would simply define project(::Type{Real}, dx::Quaternion) = ...
• It's also relatively clean (only one- and two-argument functions), compared to WIP: project implementation #380 where there are 3-arg functions. On the other hand calling project now also needs an info kwarg which stores the information about the primal x, such as size or element type.
• These also seem to compose relatively naturally. I.e. preproject returns a nested NamedTuple of required info. Similarly the project can just peel off the necessary info and pass it on to the inner project call.

There are still some things I need to address from @mcabbott's comments on #382, but just putting it out here for discussion.

IMO this is the best way forward of the three PRs. Thoughts?

[codecov-commenter]

Codecov Report

Merging #383 (d8848f5) into master (4b1a2f6) will increase coverage by 0.60%.
The diff coverage is 97.43%.

@@            Coverage Diff             @@
##           master     #383      +/-   ##
==========================================
+ Coverage   89.12%   89.73%   +0.60%     
==========================================
  Files          14       15       +1     
  Lines         561      604      +43     
==========================================
+ Hits          500      542      +42     
- Misses         61       62       +1     

Impacted Files	Coverage Δ
src/ChainRulesCore.jl	100.00% <ø> (ø)
src/projection.jl	97.43% <97.43%> (ø)
src/differentials/thunks.jl	94.89% <0.00%> (+0.21%)	⬆️

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[mzgubic]

closed in favour of #385