[sgen] Generational mono g hashtable

[sgen] Generational mono g hashtable

We mark cards when storing in hashtables. During minors, instead of scanning the entire tables, we only scan the sections for the dirty cards. In order to simplify the implementation we split into separate key/value tables.

To achieve this, instead of using an user descriptor for the hashtable structure, we directly register the tables and we create a new type of root descriptor for it to teach sgen how to find pointers in it. In addition to the precise scan of the root, which is done during majors, we also support a remset scan for it. We expect all roots registered as ROOT_TYPE_WBARRIER, to use this new descriptor (ROOT_DESC_VECTOR). In order for a root descriptor to be in the ROOT_TYPE_WBARRIER category, it has to have a defined remset scan mode.