Switch from Dict to `MultipleRefElemData` for hybrid mesh `RefElemData`

[jlchan]

Switching to a representation of rds::MultipleRefElemData (a wrapped NamedTuple type) instead of rds::LittleDict. This is more consistent with the representation of array components. For example, this PR changes the syntax to rds.Tri.Dr instead of rds[Tri()].Dr, which is consistent with the access of MeshData fields (e.g., md.x.Tri).

This PR also drops OrderedCollections.jl from the dependencies.

[codecov[bot]]

Codecov Report

Merging #116 (56e73ff) into jc/pre-0.16 (7a91cec) will decrease coverage by 0.28%. The diff coverage is 80.43%.

:exclamation: Current head 56e73ff differs from pull request most recent head 3b87508. Consider uploading reports for the commit 3b87508 to get more accurate results

@@               Coverage Diff               @@
##           jc/pre-0.16     #116      +/-   ##
===============================================
- Coverage        97.19%   96.91%   -0.28%     
===============================================
  Files               25       25              
  Lines             2748     2758      +10     
===============================================
+ Hits              2671     2673       +2     
- Misses              77       85       +8     

Impacted Files	Coverage Δ
src/hybrid_meshes.jl	90.97% <80.43%> (-5.78%)	:arrow_down:

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[jlchan]

FYI @Davknapp and @jmark. This shouldn't impact anything in Trixi.jl directly, but it's a breaking change concerning hybrid meshes in StartUpDG.jl. Please let me know if you have any comments.

[jlchan]

Closing in favor of https://github.com/jlchan/StartUpDG.jl/pull/117 (PR directly to main)