Use structured bindings

[christophebedard]

We use a lot of std::pair<>, so this simplifies the unpacking code. See: https://en.cppreference.com/w/cpp/language/structured_binding.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (rolling@f7ead0f). Click here to learn what that means.

Files	Patch %	Lines
email/src/service_client.cpp	0.00%	0 Missing and 2 partials ⚠️
email/src/service_server.cpp	0.00%	0 Missing and 1 partial ⚠️
rmw_email_cpp/src/rmw_request.cpp	0.00%	1 Missing ⚠️
rmw_email_cpp/src/rmw_response.cpp	0.00%	1 Missing ⚠️
rmw_email_cpp/src/take.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             rolling     #326   +/-   ##
==========================================
  Coverage           ?   35.99%           
==========================================
  Files              ?       87           
  Lines              ?     2834           
  Branches           ?      704           
==========================================
  Hits               ?     1020           
  Misses             ?     1257           
  Partials           ?      557           

Flag	Coverage Δ
email	60.96% <0.00%> (?)
rmw-email-cpp	0.93% <0.00%> (?)

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