Performance improvements in class_methods.rb

[guille-sage]

Makes a series of performance improvements to the methods defined by #attr_accessor_type. In most cases this will make the initial loading when defining a class a bit slower but make parsing and dumping faster.

• Use a shared ValueHelper singleton that is shared by all closures
• Remove calls to #get_attribute: use the cka closure directly
• Pre-compute getter and setter symbols (:"@name" and :"@name=")
• Define smaller methods, changing the logic depending on the type and other things known at definition time. This makes the runtime checks leaner. For example, the old code generated methods that had to check on every run whether the attribute was one_of. This is solved by checking before defining the method.

Already showing decent improvements in the benchmarks:

bench	median before (s)	median after (s)
deep_100_deserialize	0.0460s	0.0367s
deep_100_serialize	0.0260s	0.0176s
deep_10_deserialize	0.0048s	0.0035s
deep_10_serialize	0.0027s	0.0018s
deep_500_deserialize	0.2439s	0.1897s
deep_500_serialize	0.1412s	0.1004s
flat_large_deserialize	0.5162s	0.4539s
flat_large_serialize	0.1532s	0.1169s
flat_medium_deserialize	0.0485s	0.0662s
flat_medium_serialize	0.0158s	0.0150s
flat_small_deserialize	0.0054s	0.0062s
flat_small_serialize	0.0015s	0.0015s
hash_deserialize	0.0461s	0.0258s
hash_serialize	0.0350s	0.0231s
micro_from_hash	0.4109s	0.2670s
micro_from_json	0.7753s	0.6003s
micro_to_hash	0.3552s	0.2282s
micro_to_json	0.3935s	0.2758s
nested_large_deserialize	0.7513s	0.6480s
nested_large_serialize	0.4686s	0.2864s
nested_medium_deserialize	0.0723s	0.0552s
nested_medium_serialize	0.0375s	0.0239s
nested_small_deserialize	0.0083s	0.0067s
nested_small_serialize	0.0041s	0.0027s