refactor: add fast branch if the string is short enough to not be truncated

[ClementTsang]

Description

A description of the change, what it does, and why it was made. If relevant (such as any change that modifies the UI), please provide screenshots of the changes:

Another branch to skip slicing/iteration if the string is short enough compared to the width given that it must fit. This makes it fast to have a small string and/or a lot of space to accommodate it, which is likely the most usual case.

The faster_ascii below shows benchmark speed of each function (original is the implementation before #1330).

Issue

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Closes: #

Testing

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If this is a code change, please also indicate which platforms were tested:

• Windows
• macOS
• Linux

Checklist

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• Areas your change affects have been linted using rustfmt (cargo fmt)
• The change has been tested and doesn't appear to cause any unintended breakage
• Documentation has been added/updated if needed (README.md, help menu, doc pages, etc.)
• The pull request passes the provided CI pipeline
• There are no merge conflicts
• If relevant, new tests were added (don't worry too much about coverage)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a93521d) 31.77% compared to head (d111d07) 31.77%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1333   +/-   ##
=======================================
  Coverage   31.77%   31.77%           
=======================================
  Files          97       97           
  Lines       16978    16978           
=======================================
  Hits         5395     5395           
  Misses      11583    11583           

Flag	Coverage Δ
macos-12	33.55% <100.00%> (ø)
ubuntu-latest	33.35% <100.00%> (ø)
windows-2019	33.58% <100.00%> (ø)

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