implement unnest function

[jimexist]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

add unnest function that converts the argument into 0 or more rows.

1. the argument has to be of list type
2. the argument can be array literal or an array of list, in which case the expanded rows are unioned together

Describe the solution you'd like

similar to https://www.postgresql.org/docs/current/functions-array.html

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
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[lightjacket]

This is a duplicate of #212

[vincev]

Added some updates in #212.