Optimize string concatenation using << operator

[tagliala]

This change enhances the efficiency of HTML string construction, by replacing += operator with <<.

The shovel operator (<<) modifies strings in-place, reducing memory allocation and improving overall performance compared to the += operator.

Test Case

Ruby 3.3.5 x64 on M1 Pro

Arbre::Context.new { div { |d| d.ul { li } } }.to_s

Before

--- Memory Profiler ---

Total allocated: 4624 bytes (80 objects)
Total retained:  0 bytes (0 objects)

allocated memory by gem
-----------------------------------
      4624  arbre/lib

allocated memory by file
-----------------------------------
      2560  arbre/html/tag.rb
      1120  arbre/element/builder_methods.rb
       400  arbre/element_collection.rb
       280  arbre/element.rb
       264  arbre/context.rb

--- Benchmark: IPS ---

                test     50.546k (± 1.4%) i/s -    254.550k in   5.037005s

--- Benchmark: Memory ---

                test     6.208k memsize (     0.000  retained)
                        99.000  objects (     0.000  retained)
                        29.000  strings (     0.000  retained)

After

--- Memory Profiler ---

Total allocated: 3840 bytes (64 objects)
Total retained:  0 bytes (0 objects)

allocated memory by gem
-----------------------------------
      3840  arbre/lib

allocated memory by file
-----------------------------------
      1776  arbre/html/tag.rb
      1120  arbre/element/builder_methods.rb
       400  arbre/element_collection.rb
       280  arbre/element.rb
       264  arbre/context.rb

--- Benchmark: IPS ---

                test     52.205k (± 1.6%) i/s -    265.659k in   5.090028s

--- Benchmark: Memory ---

                test     5.424k memsize (     0.000  retained)
                        83.000  objects (     0.000  retained)
                        19.000  strings (     0.000  retained)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.31%. Comparing base (64aaf01) to head (34dc8f2).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #617      +/-   ##
==========================================
- Coverage   94.32%   94.31%   -0.02%     
==========================================
  Files          17       17              
  Lines         458      457       -1     
==========================================
- Hits          432      431       -1     
  Misses         26       26              

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[tagliala]

I ran the activeadmin test suite against this branch, all green

[javierjulio]

Thanks!