Add aggregate functions to `ExprTrait`

[timothee-haudebourg]

Motivation

Aggregate functions are only defined as Expr methods. Using one of them currently requires wrapping the expression inside a Expr::expr(...) to get an Expr. For instance: Expr::expr(Expr::case(...)).sum().

Proposed Solutions

By adding aggregate functions to the ExprTrait trait, it would be possible to simply write Expr::case(...).sum(), which is much more idiomatic.