Realm: Add support for multiple fields in indirect copies

[apryakhin]

We need to support use cases for indirect copies when user provides indirection coordinates as separate fields, a mix of const and non-const fields. This for example will enable users to possible reduce memory footprint by not assembling a complete indirection that is required today.

[lightsighter]

Adding myself too since this will require Legion development work as well.

[apryakhin]

@magnatelee