Improve performance for sequential combine operations (task_limit=1)

[vxgmichel]

This affects:

• concat
• flatten
• concatmap
• flatmap
• map

This is a 7x performance gain when the async function provided to map does a simple sleep(0).

See discussion in #130

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (8201a79) to head (20b684f).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #131   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           15        15           
  Lines         1167      1179   +12     
=========================================
+ Hits          1167      1179   +12     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.