Make Sequel::Migrator.migrator_class public

migrator_class is the only way to auto detect which migration filename style is being used without duplicating the same code. SequelRails and other integration libraries could benefit from this being public and usable externally.
Add specs for Migrator.migrator_class

Now that it is public, it should have specs.
Don't reference core extensions in LiteralString RDoc
Support composite keys in the tree plugin

Because the tree plugin relied mostly on the associations support,
composite keys were already supported for the most part.  The only
features that didn't support them where methods dealing with
determining the root nodes.

While here, do some significant refactoring of the integration specs
so they can be shared by both the single key and composite key case.
Support composite keys in the rcte_tree plugin

This was a lot more work than adding composite key support to the
tree plugin, since there was a bunch of code that assumed the keys
were symbols.