Backport: Implement calculus operations for ConstantSpace

[codecov[bot]]

Codecov Report

Patch coverage: 50.00% and project coverage change: -0.01% :warning:

Comparison is base (c398dc9) 71.27% compared to head (45bbfab) 71.27%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## backport_0.8 #597 +/- ## ================================================ - Coverage 71.27% 71.27% -0.01% ================================================ Files 81 81 Lines 8592 8594 +2 ================================================ + Hits 6124 6125 +1 - Misses 2468 2469 +1 ``` | [Files Changed](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/597?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation) | Coverage Δ | | |---|---|---| | [src/Spaces/ConstantSpace.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/597?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL1NwYWNlcy9Db25zdGFudFNwYWNlLmps) | `51.30% <50.00%> (-0.03%)` | :arrow_down: |

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