Allow => syntax for named cursor arguments in plpgsql.

Allow => syntax for named cursor arguments in plpgsql.

We've traditionally accepted "name := value" syntax for
cursor arguments in plpgsql.  But it turns out that the
equivalent statements in Oracle use "name => value".
Since we accept both forms of punctuation for function
arguments, it makes sense to do the same here.

Author: Pavel Stehule 
Reviewed-by: Gilles Darold 
Discussion: https://postgr.es/m/CAFj8pRA3d0ARQEMbABa1n6q25AUdNmyO8aGs56XNf9pD4sRMjQ@mail.gmail.com