feat: add u.Future and u.FanIn helpers

[moul]

No description provided.

[github-actions]

Expected release notes (by @moul-bot)

features:
add FanIn (d3a4840)
add Future (8cb7ee7)

others (will not be included in Semantic-Release notes):
update github.com/tailscale/depaware commit hash to 77d1e97 (6fc379a)

• I hereby acknowledge these release notes are 🥙 AWESOME 🥙

[codecov]

Codecov Report

Merging #32 (d3a4840) into master (f809d12) will increase coverage by 1.89%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #32      +/-   ##
==========================================
+ Coverage   70.76%   72.66%   +1.89%     
==========================================
  Files          11       11              
  Lines         260      278      +18     
==========================================
+ Hits          184      202      +18     
  Misses         47       47              
  Partials       29       29              

Flag	Coverage Δ
unittests	72.66% <100.00%> (+1.89%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
concurrency.go	100.00% <100.00%> (ø)
pattern.go	81.81% <100.00%> (+21.81%)	⬆️

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[github-actions]

🎉 This PR is included in version 1.20.0 🎉

The release is available on GitHub release

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