Make immer::box work with forward declared types

[arximboldi]

Thanks @alex-sparus for reporting and fixing this one!

Based on: https://github.com/alex-sparus/immer/commit/c29e7ff9ac8478e78ba568d3d74a7a7c937456f9

[codecov-commenter]

Codecov Report

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

Project coverage is 90.53%. Comparing base (701e125) to head (6d22844).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #282 +/- ## ======================================= Coverage 90.53% 90.53% ======================================= Files 120 120 Lines 12147 12151 +4 ======================================= + Hits 10997 11001 +4 Misses 1150 1150 ```

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