[WIP/RFC] Refactor IR around Terms.jl

[HarrisonGrodin]

Terms.jl will mostly replace the IR structure (e.g. Operation), leaving other data structures (e.g. Variable) the same until future PRs.

TODO:

• Swap Operation for Term
• Swap Constant(x) for @term(x)
• Update algorithms
• Update README

[codecov]

Codecov Report

Merging #76 into master will decrease coverage by 0.53%.
The diff coverage is 82.05%.

@@            Coverage Diff            @@
##           master     #76      +/-   ##
=========================================
- Coverage   80.43%   79.9%   -0.54%     
=========================================
  Files          11      11              
  Lines         414     413       -1     
=========================================
- Hits          333     330       -3     
- Misses         81      83       +2

Impacted Files	Coverage Δ
src/ModelingToolkit.jl	75% <ø> (ø)	⬆️
src/variables.jl	72.28% <ø> (-1.05%)	⬇️
src/systems/diffeqs/diffeqsystem.jl	82.92% <100%> (ø)	⬆️
src/systems/nonlinear/nonlinear_system.jl	96.96% <100%> (ø)	⬆️
src/differentials.jl	79.54% <100%> (ø)	⬆️
src/operations.jl	70% <50%> (-5.87%)	⬇️
src/utils.jl	82.97% <80.64%> (-5.26%)	⬇️
src/function_registration.jl	0% <0%> (-22.23%)	⬇️

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[HarrisonGrodin]

All simple features now work using Term; the more in-depth procedures (e.g. system construction) still need to be translated.

[HarrisonGrodin]

Closing this PR and bringing back changes in smarter, more targeted PRs. Although it initially seemed that exposing the use of Terms.jl would be ideal, it soon became clear that this is hard to use and develop with.

After making some of the more structural changes that accidentally became part of this, I'll open a PR which only changes the internals of Operation, ideally without changing the API. This leaves us the benefit of speed and compatibility with other Terms-based packages without requiring direct use of the abstract viewpoint taken in Terms.