Verify preflighting of nested expressions using AST visitation.

Remove logic for splitting queries by string.  Now the evaluation is
handled by a nested column resolver, and the mixed-mode expressions
are preflighted by examining the parsed abstract syntax tree for
the query expression.

Also remove `_ensure_spacing`, no longer used.

This logic was copied from pd.DataFrame.query, and the accompanying
comment said that it was to handle an occasional case where
`self.loc[b]` would raise an error on a multi-dimensional `b`, but
`self[b]` would succeed.  I can't cause this error with `.loc` anymore,
so the code coverage complains about the unexercised exception
clause.  Removing it since the limitation that inspired it seems
to be gone now.

Prevent users from depending on the public interface, and also
from assuming that a `NestedSeries` has a nest within it, since
that is the existing meaning of that prefix.