FIX: miscellaneous transformer fixes to get JSON serialization working

[klauer]

Closes #28

• ArrayInitialElementCount is wrapped into ArrayInitialElement now
• GlobalVariableDeclarations was mistakenly using VariableAttributes instead of GlobalVariableAttributes
• Remove more unused items in blark.transform
• Clean up round-trip testing and enable roundtrip serialization tests
• Add apischema to dev requirements
• VariableAttributes et al are now IntFlags (important for serialization)
• Fixed ArraySpecification to include FunctionCall
• Expanded Constant into a Union of valid types, as it's not equivalent to Literal (aliased out of laziness previously?)
• Rearranged CLI to get test coverage more easily

Some notes:

• (De)Serialization from/to JSON is slower than expected even - ample room for improvement down the line, I suppose.
  • Have not dug into details yet to see where the slowdown comes in

[codecov-commenter]

Codecov Report

Merging #42 (1a09e0c) into master (4ad8c51) will increase coverage by 4.95%.
The diff coverage is 86.84%.

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
+ Coverage   66.96%   71.92%   +4.95%     
==========================================
  Files          16       18       +2     
  Lines        3687     3754      +67     
==========================================
+ Hits         2469     2700     +231     
+ Misses       1218     1054     -164     

Impacted Files	Coverage Δ
blark/__main__.py	0.00% <0.00%> (ø)
blark/parse.py	62.65% <76.92%> (+22.65%)	⬆️
blark/tests/conftest.py	83.63% <79.54%> (-16.37%)	⬇️
blark/main.py	86.66% <86.66%> (ø)
blark/tests/test_parsing.py	93.33% <90.90%> (+12.08%)	⬆️
blark/tests/test_cli.py	92.50% <92.50%> (ø)
blark/tests/test_transformer.py	100.00% <100.00%> (+4.09%)	⬆️
blark/transform.py	98.96% <100.00%> (+0.34%)	⬆️
blark/util.py	82.06% <0.00%> (+1.37%)	⬆️
... and 4 more

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