gccrs: ast: Add new Expr::Kinds

gccrs: ast: Add new Expr::Kinds

Collapses all of the OperatorExprs into Expr instead of first having to check for OperatorExpr and
then check for each OperatorExpr::Kind.

gcc/rust/ChangeLog:

	* ast/rust-ast.h: Add new Expr::Kinds.
	* ast/rust-expr.h: Implement missing get_expr_kind(), Add get_function_expr_ptr()