feat: fix generalized indexing

[thofma]

• make A[i::Int, js::Array] return a Vector and support views

[thofma]

Split off from #1565, since it is easier this way.

[thofma]

This is breaking, but I want to add it in this 0.37.* round. Or maybe not.

[codecov]

Codecov Report

Attention: 17 lines in your changes are missing coverage. Please review.

Comparison is base (14b8c2b) 87.17% compared to head (cb43fd5) 86.93%.
Report is 9 commits behind head on master.

Files	Patch %	Lines
src/Matrix.jl	15.78%	16 Missing ⚠️
src/generic/Matrix.jl	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1585      +/-   ##
==========================================
- Coverage   87.17%   86.93%   -0.24%     
==========================================
  Files         115      115              
  Lines       29565    30075     +510     
==========================================
+ Hits        25773    26147     +374     
- Misses       3792     3928     +136     

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[lgoettgens]

This is breaking, but I want to add it in this 0.37.* round. Or maybe not.

If this is breaking, we cannot include it in 0.37.2. Instead, this has to wait for 0.38.0

[thofma]

Yeah, 0.38.0 it is