Refactor and simplify plugin API

[solnic]

• Replace deeply nested plugin registries with a single one
• Add a backward-compatibility Resolver
• Reduce and fix the number of requires

Specs run ~4 seconds faster on my machine with this so I suspect it's
going to have an impact in applications too.

You can now easily inspect what plugins have been loaded:

> ROM.plugin_registry.keys
["schema.timestamps", "relation.registry_reader", "relation.instrumentation", "command.schema", "command.timestamps", "sql.relation.instrumentation", "sql.relation.auto_restrictions", "sql.relation.nullify", "sql.relation.pagination", "sql.command.associates", "sql.relation.pg_explain", "sql.relation.pg_full_text_search"]

> ROM.plugin_registry["sql.relation.pagination"]
#<ROM::Plugin:0x00007f66761f8798 @name=:pagination, @mod=ROM::SQL::Plugin::Pagination, @type=:relation, @adapter=:sql>