Improve type-inference in broadcasting

[jishnub]

Using broadcast inside a broadcast operation leads to recursion, which impedes type inference. This PR changes the inner operation to a map, which makes the broadcast fully type inferred.
On master

julia> B = BlockArray([1:3;], [2,1]);

julia> A = zeros(3);

julia> @btime $A .= $B;
  1.075 μs (14 allocations: 816 bytes)

This PR

julia> @btime $A .= $B;
  454.323 ns (9 allocations: 624 bytes)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.58%. Comparing base (ea33af1) to head (4148544).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #351      +/-   ##
==========================================
+ Coverage   92.66%   93.58%   +0.92%     
==========================================
  Files          16       16              
  Lines        1499     1497       -2     
==========================================
+ Hits         1389     1401      +12     
+ Misses        110       96      -14     

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