Fixes incorrect handling of SID 0 as symbol text

[popematt]

Issue #, if available:

None.

Description of changes:

• Changes the system symbol constants to be Option<&str> and changes SID $0 to None rather than "$0"
• Propagate SID $0 through the text writers as a RawSymbolToken or RawSymbolTokenRef rather than forcing all symbols to be converted to text.
• Updates round_trip_skip_list() in element_test_vector.rs to no longer skip SID 0 test cases.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.20 🎉

Comparison is base (049da35) 88.69% compared to head (86f3336) 88.90%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #515      +/-   ##
==========================================
+ Coverage   88.69%   88.90%   +0.20%     
==========================================
  Files          81       81              
  Lines       12888    12885       -3     
==========================================
+ Hits        11431    11455      +24     
+ Misses       1457     1430      -27     

Impacted Files	Coverage Δ
src/symbol_table.rs	91.66% <100.00%> (+13.33%)	⬆️
src/system_reader.rs	85.42% <100.00%> (ø)
src/text/raw_text_writer.rs	96.75% <100.00%> (+0.54%)	⬆️
src/text/text_formatter.rs	58.86% <100.00%> (+0.24%)	⬆️
src/text/text_writer.rs	95.45% <100.00%> (+3.14%)	⬆️

... and 3 files with indirect coverage changes

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[zslayton]

Excellent! I see a couple of other things that would be nice to address while you're making changes to this code if you don't mind.

[zslayton]

🚀