Exec-next: legacy instance methods

[rmosolgo]

All the necessary work is already here to support legacy instance methods, it just needs to be prettied up.

TODO:

• Out of curiosity, benchmark it. I suspect that execute_next with resolve_legacy_instance_method: true is still faster than .execute.
• Document this new option
• Make sure docs clearly state that the implicit default behavior has changed.
• Support legacy implicit default? No, but it can be hard coded.

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Benchmark results

In the "large benchmark" in this gem (a basic loop over 1000 objects), using legacy_instance_method adds an allocation per object and, in that example, it's 10% slower than using hash_key:

Comparison:
         exec_next hash_key:      211.2 i/s
    exec_next legacy_method:      193.2 i/s - 1.09x  slower
    
       legacy_exec hash_key:       14.7 i/s - 14.33x  slower

They're both 14x faster than legacy execution 😬

Memory:

Hash Key:      Total allocated: 1244256 bytes (4455 objects)
Legacy Method: Total allocated: 1292296 bytes (5456 objects)

1001 more objects: one array and 1000 GraphQL object type instances.