Make type aliases constant

[codecov[bot]]

Codecov Report

Base: 84.12% // Head: 84.12% // No change to project coverage :thumbsup:

Coverage data is based on head (9f08f07) compared to base (8f009b2). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #54 +/- ## ======================================= Coverage 84.12% 84.12% ======================================= Files 5 5 Lines 63 63 ======================================= Hits 53 53 Misses 10 10 ``` | [Impacted Files](https://codecov.io/gh/JuliaGPU/Adapt.jl/pull/54?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaGPU) | Coverage Δ | | |---|---|---| | [src/wrappers.jl](https://codecov.io/gh/JuliaGPU/Adapt.jl/pull/54?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaGPU#diff-c3JjL3dyYXBwZXJzLmps) | `75.00% <ø> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaGPU). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaGPU)

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[maleadt]

LGTM, thanks!