Memoize field maps for Objects and Interfaces

[fd]

No description provided.

[coveralls]

Coverage increased (+0.03%) to 82.25% when pulling 007208e on mrhenry:memoize-fieldmaps into 9b5a661 on graphql-go:master.

[fd]

benchmark                       old ns/op       new ns/op     delta
BenchmarkListQuery_1-4          349864          197375        -43.59%
BenchmarkListQuery_100-4        3534198         724916        -79.49%
BenchmarkListQuery_1K-4         32966933        5349084       -83.77%
BenchmarkListQuery_10K-4        308862819       52980057      -82.85%
BenchmarkListQuery_100K-4       2255458560      372151071     -83.50%
BenchmarkWideQuery_1_1-4        159626          129124        -19.11%
BenchmarkWideQuery_10_1-4       960218          335290        -65.08%
BenchmarkWideQuery_100_1-4      62290987        2214388       -96.45%
BenchmarkWideQuery_1K_1-4       6342374529      21798083      -99.66%
BenchmarkWideQuery_1_10-4       196672          151809        -22.81%
BenchmarkWideQuery_10_10-4      2356517         489904        -79.21%
BenchmarkWideQuery_100_10-4     190874887       3423580       -98.21%
BenchmarkWideQuery_1K_10-4      20784765533     35847946      -99.83%

[chris-ramon]

this is very awesome, def. lgtm thanks a lot @fd 👍