Impl. an "at setter" that works with immer::table

[colugomusic]

This allows us to create a cursor to an immer::table element i.e.

struct item { int id; }
struct model {
  immer::table<item> items;
};
...
lager::reader<item> item_123 = store[&model::items][123].zoom(lager::lenses::or_default);

This is currently a compile error because immer::table has no set operation, so the first implementation of at_setter_impl would be selected leading to the invalid assignment.

[arximboldi]

This one seems to fail...

[arximboldi]

Ah but it's not your fault. Restarting CI...

[arximboldi]

CI is failing because of outdated actions. Can you rebase on latest master so we can see a green build/check?

[arximboldi]

Other than that, thank you a lot for this contribution, this is very useful... table was introduced relatively recently and Lager hadn't been updated to match yet.

[codecov-commenter]

Codecov Report

Merging #178 (1890eae) into master (cd62f72) will not change coverage.
The diff coverage is n/a.

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@@           Coverage Diff           @@
##           master     #178   +/-   ##
=======================================
  Coverage   94.47%   94.47%           
=======================================
  Files          80       80           
  Lines        2698     2698           
=======================================
  Hits         2549     2549           
  Misses        149      149           

Impacted Files	Coverage Δ
lager/lenses/at.hpp	100.00% <ø> (ø)