Remove namespace directives from util headers

[aobolensk]

• drop using namespace ppc::task from func/perf test headers
• drop using namespace ppc::performance from perf test header
• qualify names in util helpers with their namespaces

[github-actions]

Coverage report is available for download
here

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.57%. Comparing base (8b3e8fb) to head (7479dea).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #520   +/-   ##
=======================================
  Coverage   94.57%   94.57%           
=======================================
  Files          11       11           
  Lines         350      350           
  Branches      117      117           
=======================================
  Hits          331      331           
  Misses          5        5           
  Partials       14       14           

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