symbolic save idxs, new observed

[xtalax]

Merge order is:

1. https://github.com/SciML/SymbolicIndexingInterface.jl/pull/7
2. https://github.com/SciML/SciMLBase.jl/pull/392 (This)
3. https://github.com/SciML/OrdinaryDiffEq.jl/pull/1867
4. https://github.com/SciML/ModelingToolkit.jl/pull/2071

Am trying to test each part seperately but a lot of this is very tightly coupled, I will test it composed in the MTK PR locally with all dev'd and notify when that passes

[codecov[bot]]

Codecov Report

Merging #392 (bb17abc) into master (c2bb35d) will decrease coverage by 13.03%. The diff coverage is 12.50%.

:exclamation: Current head bb17abc differs from pull request most recent head 59dd393. Consider uploading reports for the commit 59dd393 to get more accurate results

@@             Coverage Diff             @@
##           master     #392       +/-   ##
===========================================
- Coverage   52.25%   39.22%   -13.03%     
===========================================
  Files          46       46               
  Lines        3554     3602       +48     
===========================================
- Hits         1857     1413      -444     
- Misses       1697     2189      +492     

Impacted Files	Coverage Δ
src/SciMLBase.jl	46.66% <ø> (ø)
src/solutions/nonlinear_solutions.jl	38.46% <ø> (-23.08%)	:arrow_down:
src/solutions/optimization_solutions.jl	34.78% <ø> (-19.57%)	:arrow_down:
src/solutions/rode_solutions.jl	66.66% <ø> (ø)
src/solutions/solution_interface.jl	30.80% <0.00%> (-28.10%)	:arrow_down:
src/symbolic_utils.jl	15.38% <0.00%> (-50.19%)	:arrow_down:
src/utils.jl	54.48% <60.00%> (-7.11%)	:arrow_down:
src/solutions/dae_solutions.jl	19.56% <100.00%> (ø)
src/solutions/ode_solutions.jl	72.07% <100.00%> (-22.33%)	:arrow_down:

... and 22 files with indirect coverage changes

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[ChrisRackauckas]

Isn't this needed in every solution type for the indexing?

[xtalax]

is build_explicit_observed_function used for every system type? If so then yes, but I think it would require updating every __init__ to feed through the symbol map. I will have a think about this

[YingboMa]

Are the test failures related?

[xtalax]

Remaining failures look unrelated