Extend built-in completion

[rrudakov]

Complete keywords and local bindings in for and doseq forms.

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[bbatsov]

Looks good.

I was thinking at some point we might also add completion for the imported namespaces and their aliases.

[rrudakov]

I was thinking at some point we might also add completion for the imported namespaces and their aliases.

Good idea, I'll try to implement it.