Unused grammar rule clean-up and grammar coverage tests

[klauer]

(This is part of a larger branch which got to be unmanageably large - breaking it up into separate PRs)

Fixes:

• Fixes incorrect escape character for double quotes in double-byte strings ("$"")
• Don't add more parentheses than required when formatting dataclasses back to code (specifically for ObjectInitializerArray)

Clean-up:

• Removed unused grammar rules structured_var_declaration, function_var_decl, step_*, etc.
• Removed grammatical alias structure_type_name from var_spec (since simple_spec covers it just fine)

Test suite:

• More coverage for grammar rules in test_transformer
• Coverage check to ensure that all known rules are hit (terminals are a bit buggy there, but I'm not too worried about it)
  • This is opt-in, setting BLARK_CHECK_GRAMMAR_COVERAGE=1 in the environment prior to running the test suite (since you might only run part of the test suite and it'd be annoying for the coverage check to fail then)

[codecov-commenter]

Codecov Report

Merging #86 (58f5bc6) into master (16498b7) will increase coverage by 0.1%.
The diff coverage is 94.7%.

@@           Coverage Diff            @@
##           master     #86     +/-   ##
========================================
+ Coverage    80.7%   80.8%   +0.1%     
========================================
  Files          26      26             
  Lines        5510    5559     +49     
========================================
+ Hits         4447    4493     +46     
- Misses       1063    1066      +3     

Files	Coverage Δ
blark/tests/test_transformer.py	94.7% <100.0%> (+1.2%)	⬆️
blark/transform.py	98.9% <100.0%> (+<0.1%)	⬆️
blark/util.py	83.8% <81.2%> (-0.2%)	⬇️

... and 1 file with indirect coverage changes