Added integral function

[the-real-stiven]

Native implementation of integral() Graphite function.

Speed improvement:

---------- Native targets/integral.json Latencies ----------
Mean: 8.715571ms
50th percentile: 7.740028ms
95th percentile: 15.169597ms
99th percentile: 21.068161ms
Max: 23.105963ms
Success: 100%
Errors: []

---------- Graphite (Python) targets/integral.json Latencies ----------
Mean: 27.9726ms
50th percentile: 25.656504ms
95th percentile: 40.603139ms
99th percentile: 52.347557ms
Max: 141.412371ms

Success: 100%
Errors: []

---------- Speed Improvement ----------
Mean: x3.2
50th percentile: x3.3
95th percentile: x2.7
99th percentile: x2.5
Max: x6.1