use BlockedTuple for default label_dest

[ogauthe]

When label_dest is not provided, contract automatically determines it from labels1 and labels2 and returns it. It currently output it as a Tuple. This PR makes it return a BlockedTuple{2}.

This PR also add many sanity checks.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.88%. Comparing base (1e90db0) to head (59159d7).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #64      +/-   ##
==========================================
+ Coverage   94.63%   94.88%   +0.25%     
==========================================
  Files          14       14              
  Lines         429      450      +21     
==========================================
+ Hits          406      427      +21     
  Misses         23       23              

Flag	Coverage Δ
docs	0.00% <0.00%> (ø)

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[mtfishman]

Looks good, thanks!