Add `read_at_unchecked()` specialization for `map_adaptor`

[DeveloperPaul123]

Attempts to address #94

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (89d6caa) 97.70% compared to head (32bdf01) 97.70%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #121 +/- ## ======================================= Coverage 97.70% 97.70% ======================================= Files 67 67 Lines 2398 2398 ======================================= Hits 2343 2343 Misses 55 55 ``` | [Files Changed](https://app.codecov.io/gh/tcbrindle/flux/pull/121?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tristan+Brindle) | Coverage Δ | | |---|---|---| | [include/flux/op/map.hpp](https://app.codecov.io/gh/tcbrindle/flux/pull/121?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tristan+Brindle#diff-aW5jbHVkZS9mbHV4L29wL21hcC5ocHA=) | `93.33% <ø> (ø)` | |

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[tcbrindle]

Thanks @DeveloperPaul123, looks great!

Fixes #94