Bugfix: `restrict_to_field` creates copies for `BlockMultiFieldStyle`

[JordiManyer]

This is a fix for a bug created by the following PR:

• https://github.com/JuliaLinearAlgebra/ArrayLayouts.jl/pull/219
• https://github.com/JuliaArrays/BlockArrays.jl/issues/236

The behavior of x[Block(ib)] changed, creating copies of the data. It's intentional, so we need to resolve it from our side.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.17%. Comparing base (e688660) to head (cfb2bf7).

:exclamation: Current head cfb2bf7 differs from pull request most recent head 369de95. Consider uploading reports for the commit 369de95 to get more accurate results

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #993 +/- ## ======================================= Coverage 88.17% 88.17% ======================================= Files 176 176 Lines 22021 22021 ======================================= Hits 19416 19416 Misses 2605 2605 ```

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