Switch from a mutation loop to a generator in a make_idx path

[LilithHafner]

The new version is 1.5x faster for constructing a ComponentArray with 100 fields each with a 10 element vector.

Benchmarks:

julia> using ComponentArrays, Chairmarks
Precompiling ComponentArrays
  1 dependency successfully precompiled in 1 seconds. 36 already precompiled.

julia> s(i) = Symbol("x"*string(i))
s (generic function with 1 method)

julia> const NT = (;(s(i) => randn(10) for i in 1:100)...);

julia> f() = ComponentArray(NT)
f (generic function with 1 method)

julia> @b f() # PR
28.292 μs (246 allocs: 63.703 KiB)

shell> git checkout main
Switched to branch 'main'
Your branch is up to date with 'upstream/main'.

julia> @b f() # main
42.834 μs (849 allocs: 84.344 KiB)

This is part of a journey to make ComponentArray(x = [1,2,3]) not use mutation so that it is compatible with Zygote.jl which is motivated in part by DARPA-ASKEM/sciml-service#141.

[jonniedie]

Oh awesome. Thanks for this. This has been a long-standing issue that I've wanted to fix, but haven't really had the time to dive into.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (94ab225) 73.99% compared to head (f2d9983) 73.92%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #242      +/-   ##
==========================================
- Coverage   73.99%   73.92%   -0.07%     
==========================================
  Files          23       23              
  Lines         746      744       -2     
==========================================
- Hits          552      550       -2     
  Misses        194      194              

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