good first PR: implement percent_rank and cume_dist built-in window functions

[jimexist]

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good first PR: implement percent rank and cume dist built-in window functions

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similar API to that of Postgres or SQLite, following the current rank and dense rank implementation, accompanied by an integration tests

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[alamb]

The idea is to add cume_dist and percent_rank as described in postgres (and elsewhere)

https://www.postgresql.org/docs/10/functions-window.html