Implement modulus operator

[aholyoke]

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Fixes #<issue_number_goes_here> 🦕

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Adds implementation of mod operator %

Currently if I try to use % in python code, it will fall back to the SA default compiler implementation which renders it as a percent sign, which is not syntactically valid in BQ. I could workaround this by using func.mod, but this isn't portable across dialects (eg. SQL Server has support for %, but not MOD)