parallel_image_crop

sh

```bash
(base) ➜  parallel_image_crop git:(master) cargo build --release
    Updating registry `https://github.com/rust-lang/crates.io-index`
 Downloading libc v0.2.43                                                       
   Compiling version_check v0.1.4                                               
   Compiling nodrop v0.1.12
   Compiling cfg-if v0.1.4
   Compiling memoffset v0.2.1
   Compiling scopeguard v0.3.3
   Compiling num-traits v0.2.5
   Compiling unicode-xid v0.1.0
   Compiling num-integer v0.1.39
   Compiling libc v0.2.43
   Compiling rayon-core v1.4.1
   Compiling rayon v1.0.2
   Compiling num-iter v0.1.37
   Compiling either v1.5.0
   Compiling adler32 v1.0.3
   Compiling byteorder v1.2.4
   Compiling bitflags v1.0.3
   Compiling lzw v0.10.0
   Compiling color_quant v1.0.1
   Compiling scoped_threadpool v0.1.9
   Compiling arrayvec v0.4.7
   Compiling crossbeam-utils v0.2.2
   Compiling proc-macro2 v0.4.9
   Compiling itertools v0.7.8
   Compiling inflate v0.4.3
   Compiling num_cpus v1.8.0
   Compiling deflate v0.7.18
   Compiling lazy_static v1.1.0
   Compiling gif v0.10.0
   Compiling crossbeam-epoch v0.3.1
   Compiling crossbeam-deque v0.2.0
   Compiling quote v0.6.5
   Compiling syn v0.14.7
   Compiling num-rational v0.1.42
   Compiling png v0.12.0
   Compiling jpeg-decoder v0.1.15
   Compiling num-derive v0.2.2
   Compiling image v0.19.0
   Compiling parallel_image_crop v0.1.0 (file:///tmp/parallel_image_crop)
warning: unused import: `std::fs::File`
 --> src/lib.rs:7:5
  |
7 | use std::fs::File;
  |     ^^^^^^^^^^^^^
  |
  = note: #[warn(unused_imports)] on by default

warning: unused import: `OsStr`
 --> src/lib.rs:9:22
  |
9 | use std::ffi::{CStr, OsStr};
  |                      ^^^^^

warning: unused import: `c_uchar`
  --> src/lib.rs:12:28
   |
12 | use libc::{size_t, c_char, c_uchar};
   |                            ^^^^^^^

warning: unused imports: `ColorType`, `ImageBuffer`, `imageops`
  --> src/lib.rs:13:27
   |
13 | use image::{GenericImage, ImageBuffer, imageops, FilterType, ColorType, ImageDecoder};
   |                           ^^^^^^^^^^^  ^^^^^^^^              ^^^^^^^^^

warning: unused import: `ImageDecoder`
  --> src/lib.rs:13:73
   |
13 | use image::{GenericImage, ImageBuffer, imageops, FilterType, ColorType, ImageDecoder};
   |                                                                         ^^^^^^^^^^^^

warning: method is never used: `into_vec`
  --> src/lib.rs:70:5
   |
70 |     fn into_vec(self) -> Vec<u8> {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: #[warn(dead_code)] on by default

    Finished release [optimized] target(s) in 46.87s
(base) ➜  parallel_image_crop git:(master) python benchmarks/test.py --batch-size=32 --num-trials=100           
python crop average over 100 trials : 0.116404447555542 +/- 0.1595486246983642 sec
rust crop average over 100 trials : 0.09679011821746826 +/- 0.0062470562722498025 sec

(base) ➜  parallel_image_crop git:(master) convert assets/lena.png  -resize 4000x4000 assets/lena.png 
(base) ➜  parallel_image_crop git:(master) ✗ python benchmarks/test.py --batch-size=32 --num-trials=100
python crop average over 100 trials : 3.173974087238312 +/- 0.2067425334971192 sec
rust crop average over 100 trials : 4.159008667469025 +/- 0.20634427332646715 sec
```