Less leakage of container type in concrete syntax tree.

[hzeller]

Reduces the leaky abstraction of handing out children() by SyntaxTreeNode, but provide operations to work on them: front(), back(), operator[].

This change does not get rid of handing out children() entirely yet, as they are used in range loops, this will be a separate change.

Reducing the abstraction leak will make it easier to change that implementation later (e.g. for issue #1523)

[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (5be583b) 92.85% compared to head (a4a5670) 92.85%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2026      +/-   ##
==========================================
- Coverage   92.85%   92.85%   -0.01%     
==========================================
  Files         357      357              
  Lines       26397    26394       -3     
==========================================
- Hits        24510    24507       -3     
  Misses       1887     1887              

Files	Coverage Δ
common/text/concrete_syntax_tree.cc	100.00% <100.00%> (ø)
common/text/concrete_syntax_tree.h	96.82% <100.00%> (+0.46%)	⬆️
common/text/tree_builder_test_util.cc	100.00% <100.00%> (ø)
common/text/tree_utils.cc	97.27% <100.00%> (-0.02%)	⬇️
common/text/tree_utils.h	100.00% <100.00%> (ø)
verilog/CST/expression.cc	96.10% <100.00%> (-0.10%)	⬇️
verilog/CST/macro.cc	100.00% <100.00%> (ø)
verilog/CST/module.cc	100.00% <100.00%> (ø)
verilog/CST/port.cc	94.36% <100.00%> (ø)
verilog/CST/seq_block.cc	100.00% <100.00%> (ø)
... and 13 more

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