Improve performance of IPM

[mateuszbaran]

This significantly improves performance of interior point Newton method. Together with some light improvements to ManifoldsBase, I got from 9ms to below 3ms on the problem from the test suite. The two biggest issues were:

1. Accessing product manifold components like Y[N, 1] is really slow. I recommend using things like Y1, Y2 = submanifold_components(N, Y) in performance-sensitive code. Maybe it would be possible to speed up Y[N, 1] but it would require tinkering with compiler heuristics which is quite difficult and depends on the version of Julia.
2. Broadcasting. It can eliminate vast majority of intermediate arrays.

Not that many potential gains remain as those 3ms are dominated by calls to Hessian and gradient of g.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.78%. Comparing base (0b05c8c) to head (2705059).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #404   +/-   ##
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  Coverage   99.78%   99.78%           
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  Files          76       76           
  Lines        8217     8226    +9     
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+ Hits         8199     8208    +9     
  Misses         18       18           

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[kellertuer]

I was not aware of the first. For the second I was maybe more focused on the math being correct. Thanks for fixing this :)

[kellertuer]

at least the sub manifold components I would not have known to use. thanks for those improvements.