Refactor: Extract to namespace

[markmandel]

Take a defn or def and move it to a new / existing namespace, and automatically use the correct use/require in the (ns) declaration.

(I'm sure this is tricky).

Combined with Extract Function, this would be very powerful.

[rlewczuk]

+1

(I realized I need this one too)

[rafaeldff]

+1 this would be incredible

[cursive-ide]

Closing this since it's a dupe of #817, which has more detail.