expr: add Union operation
pandas: union
python: union
spark: union
sql: union
Add parser tests
Add BlazeParser for scalar expressions
Move ScalarSymbol to parser
Give z a reasonable type
Add failing chained comparison test
Move exprify to parser.py
Add test for string comparison
Test string comparison with isidentical
Use literals where possible
Remove empty line
Break out test into a class and add unary sub test
Implement UnaryOp parsing
Remove unused imports in python compute
Remove extra Scalar implementation
Make Scalar constructible