Improve efficiency of (nested) StreamIndenter objects

[jsiirola]

Fixes # .

Summary/Motivation:

This PR makes two fundamental changes to the StreamIndenter:

• rely on (3) regular expressions to handle the indentation instead of breaking apart and piecing back together the strings
• change the implementation so that nested StreamIndenter instances run with constant overhead (instead of linear in the number of nested indenters)

Changes proposed in this PR:

• switch to using re for inserting the indentation
• add improved special-case handling for blank lines (so that we do not add unnecessary whitespace into blank lines)
• make nested indenters collect all indentation and then write directly to the underlying (unindented) output stream
• improve testing

Legal Acknowledgement

By contributing to this software project, I have read the contribution guide and agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.23%. Comparing base (168e961) to head (d5dd777).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3813      +/-   ##
==========================================
- Coverage   89.41%   89.23%   -0.19%     
==========================================
  Files         909      909              
  Lines      105579   105588       +9     
==========================================
- Hits        94408    94217     -191     
- Misses      11171    11371     +200     

Flag	Coverage Δ
builders	29.12% <21.42%> (+<0.01%)	⬆️
default	86.04% <100.00%> (?)
expensive	35.76% <21.42%> (?)
linux	86.73% <100.00%> (-2.46%)	⬇️
linux_other	86.73% <100.00%> (+<0.01%)	⬆️
osx	82.88% <100.00%> (+<0.01%)	⬆️
win	?
win_other	?

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