issues-398 Refactoring work with function

[ikrivosheev]

PR Info

• Closes Add ability to select from function #398

Adds

• SelectStatement::from_function - allow to use SELECT FROM func()
• impl<T> From<T> for SelectExpr where T: Into<SimpleExpr>
• impl From<FunctionCall> for SimpleExpr
• New struct FunctionCall hold function and args for call the function

Breaking Changes

• Func::* now return FunctionCall insted SimpleExpr
• Func::coalesce accept IntoIterator<Item = SimpleExpr> instead IntoIterator<Item = Into<SimpleExpr>

Changes

• Expr::expr accept Into<SimpleExpr> instead SimpleExpr
• Removed Expr::arg and Expr::args - this functions are unneeded

[tyt2y3]

Looks good at first glance. Seems like a lot of changes. Can we release this later, i.e. 0.28?