Add conditional attribute rendering

[Nitemaeric]

Context

The following issues go into the detail of the need for these changes:

• Add conditional rendering of attributes and associations #22
• Use duck-typing to avoid conditional between attributes and associations #23

Essentially, we need a way to render attributes only when they need to be included. This is useful for

Concerns and considerations

• How much of an impact does this have on performance?
• Are conditional attributes actually a good practice? Should a separate serializer be used instead?

Changes

• Add the Attribute and Association classes.
• Allow users to pass through if: and unless: Symbol or Proc values.

[Nitemaeric]

Attributes

Gem	IPS	Comparison	Allocations	Comparison
panko_serializer 0.8.3	426.260k ± 9.3%	baseline	1.016k	baseline
transmutation 0.5.1	301.750k ± 5.8%	1.41x slower	1.864k	1.83x more
active_model_serializers 0.10.15	229.911k ± 2.9%	1.85x slower	2.368k	2.33x more
jsonapi-serializer 2.2.0	211.662k ± 1.3%	2.01x slower	3.400k	3.35x more
representable 3.2.0	171.186k ± 1.3%	2.49x slower	4.304k	4.24x more
rabl 0.17.0	120.789k ± 2.5%	3.53x slower	5.856k	5.76x more
jbuilder 2.13.0	100.800k ± 2.7%	4.23x slower	2.960k	2.91x more

Has One / Belongs To

Gem	IPS	Comparison	Allocations	Comparison
panko_serializer 0.8.3	143.595k ± 2.4%	baseline	4.128k	1.03x more
transmutation 0.5.1	117.557k ± 1.7%	1.22x slower	4.024k	baseline
representable 3.2.0	87.700k ± 2.0%	1.64x slower	6.184k	1.54x more
active_model_serializers 0.10.15	66.353k ± 3.6%	2.16x slower	6.384k	1.59x more
rabl 0.17.0	64.435k ± 2.3%	2.23x slower	9.608k	2.39x more
jsonapi-serializer 2.2.0	53.159k ± 6.3%	2.70x slower	12.368k	3.07x more
jbuilder 2.13.0	44.750k ± 1.7%	3.21x slower	4.944k	1.23x more

Has Many

Gem	IPS	Comparison	Allocations	Comparison
panko_serializer 0.8.3	263.676k ± 5.6%	baseline	1.376k	baseline
transmutation 0.5.1	80.908k ± 2.6%	3.26x slower	6.048k	4.40x more
representable 3.2.0	57.544k ± 2.2%	4.58x slower	9.640k	7.01x more
active_model_serializers 0.10.15	46.549k ± 2.0%	5.66x slower	9.480k	6.89x more
jbuilder 2.13.0	43.691k ± 1.7%	6.04x slower	6.208k	4.51x more
jsonapi-serializer 2.2.0	40.179k ± 1.3%	6.56x slower	16.408k	11.92x more
rabl 0.17.0	30.535k ± 3.4%	8.64x slower	10.912k	7.93x more