7276: Remove map module from la-arena public API r=lnicola a=arzg
It’s unlikely that more items will be added to the module, so it’s simpler for users if `ArenaMap` is re-exported and the module made private.
This doesn’t compile for the same reason that #7275 doesn’t:
> This pull request doesn’t compile because dependencies on la-arena go through crates.io, so existing dependencies on the crate are referencing an old version. As such, this PR will only compile once a new la-arena version has been published.
Co-authored-by: Aramis Razzaghipour <aramisnoah@gmail.com>
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