Backport: Promote rangespace/domainspace of Conversion

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.22% :tada:

Comparison is base (42a2c04) 71.00% compared to head (7efda39) 71.23%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## backport_0.8 #539 +/- ## ================================================ + Coverage 71.00% 71.23% +0.22% ================================================ Files 81 81 Lines 8575 8577 +2 ================================================ + Hits 6089 6110 +21 + Misses 2486 2467 -19 ``` | [Files Changed](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/539?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation) | Coverage Δ | | |---|---|---| | [src/Operators/banded/Conversion.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/539?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL09wZXJhdG9ycy9iYW5kZWQvQ29udmVyc2lvbi5qbA==) | `85.93% <100.00%> (+2.06%)` | :arrow_up: | ... and [3 files with indirect coverage changes](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/539/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation)

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