Feature request: attribute scoped sequence

[kakubin]

Problem this feature will solve

As I mentioned in #1640, I want a scope for sequence.
The previous PR I created was not reviewed, so I made it as a separate gem. However, maintaining it individually seems troublesome, so I would like this to be merged.

[colinross]

Does your priority HAVE to start with 1 under each scope? if not, just use a normal (inline) sequence.

[kakubin]

@colinross
I want different sequence generators for each scope. Starting with 1 is optional, see L13.

factory_bot/lib/factory_bot/sequence.rb

Lines 5 to 13 in c0f2679

	class Sequence
	attr_reader :name
	def initialize(name, *args, &proc)
	@name = name
	@proc = proc
	options = args.extract_options!
	@value = args.first || 1