Redesign non-siu select method

[machow]

Currently select bends over backwards trying to make specifying with siu expressions, also work when just plain lambdas are used.

For example

select(_.a, _.b)
select(lambda _: _.a, lambda _: _.b)

select(lambda _: _.contains("a"))

The last statement works because the lambda will be passed a VarSelect instance. However, this has two limitations:

• it is annoying when you just want to operate on an array of column names
• it's necessary, since at the point you want to use lambdas, you should just replace _["a": "b"] with the object it creates downstream, a slice.

If we change select to...

• create intermediate objects for Call args (e.g. a slice, string, or list of those things)
• pass the array of column names to a callable

Then it opens room for people to use their own functions with select, and provides an easy to understand behavior in this case.

[machow]

See #365

[machow]

Let's remove the use of a pandas Series while we're at it. This will let us remove pandas imports from the sql verb module.