Finish SQL functions refactoring

[dreadatour]

This is a follow-up issue for the #228 and #227.

We now have SQL functions implemented in two places:

• new func module: https://github.com/iterative/datachain/tree/main/src/datachain/lib/func with support of types and signal columns
• old functions module: https://github.com/iterative/datachain/tree/main/src/datachain/sql/functions with raw sqlalchemy functions

We need to update the code to use new func module only.

@dmpetrov:

Yes, it should be imported from top level: from datachain.func import min, concat

[dmpetrov]

another requirement: we should never expose keywords like sql and sqlalchemy. it introduces quite a bit of confusion.

Users should be able implementing theirs scenario without touching these keywords and libs.