Add Len to FluentSlice, FluentMap, FluentString

[pellared]

Why

Fixes #123

What

• Add Len to FluentSlice, FluentMap, FluentString.
• Opportunistic change: Refine doc comments for Any, All, None assertions.

Checklist

• CHANGELOG.md is updated.
• README.md is updated.
• The code changes follow Effective Go.
• The code changes follow CodeReviewComments.
• The code changes are covered by tests.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d816578) 100.00% compared to head (e070248) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #124   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           15        15           
  Lines          511       529   +18     
=========================================
+ Hits           511       529   +18     

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