Update mapper variable name in PropertyCaster

[shadowhand]

The current definition has a variable named "hydrator":

interface PropertyCaster
{
    public function cast(mixed $value, ObjectMapper $hydrator): mixed;
}

It used to be that ObjectMapper was called ObjectHydrator, which is likely why this variable has the name it does. Changing this will require a major release, but I think it should be:

interface PropertyCaster
{
    public function cast(mixed $value, ObjectMapper $mapper): mixed;
}

My editor would really like this, because it doesn't think "hydrator" is a word. And of course, it would make the signature more consistent.

[frankdejonge]

This will be handled in league/object-mapper