Simplify read_only() implementation

[tcbrindle]

• Fix broken read_only() test
• Implement read_only_adaptor by inheriting from map_adaptor and adding data() in the case where Base is a contiguous_sequence

Fixes #96

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (b423b21) 97.67% compared to head (ed63880) 97.67%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #97 +/- ## ======================================= Coverage 97.67% 97.67% ======================================= Files 66 66 Lines 2237 2237 ======================================= Hits 2185 2185 Misses 52 52 ```

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