Updated notes about contributing

Travis: Use GCC9 to build on bionic for icelake compilation support

_mm512_bitshuffle_epi64_mask implementation

Cmake support added by @shohirose 

Updated benchmarks using a proper release-mode build

```
CPU:       Topology: Quad Core model: Intel Core i7-1065G7 bits: 64 type: MT MCP L2 cache: 8192 KiB
           Speed: 950 MHz min/max: 400/3900 MHz Core speeds (MHz): 1: 864 2: 958 3: 615 4: 806 5: 977 6: 704 7: 702 8: 840

    00.915 ms 0.854 ms : 64-bit BMI2 instruction set
    01.060 ms 1.033 ms : 64-bit AVX512 instruction set
    0.881 ms 0.841 ms : 32-bit BMI2 instruction set
    1.065 ms 1.102 ms : 32-bit AVX512 instruction set
    1.082 ms 4.436 ms : 64-bit BMI2 Instruction set
    0.894 ms 4.216 ms : 64-bit AVX512 Instruction set
    00.933 ms 5.062 ms : 32-bit BMI2 Instruction set
    01.075 ms 4.701 ms : 32-bit AVX512 Instruction set
    08.231 ms 6.995 ms : 64-bit BMI2 instruction set
    08.235 ms 8.252 ms : 64-bit AVX512 instruction set
    7.091 ms 6.755 ms : 32-bit BMI2 instruction set
    8.238 ms 8.264 ms : 32-bit AVX512 instruction set
    6.836 ms 33.431 ms : 64-bit BMI2 Instruction set
    6.872 ms 33.619 ms : 64-bit AVX512 Instruction set
    06.908 ms 33.352 ms : 32-bit BMI2 Instruction set
    06.839 ms 33.265 ms : 32-bit AVX512 Instruction set
    57.233 ms 57.425 ms : 64-bit BMI2 instruction set
    65.779 ms 66.391 ms : 64-bit AVX512 instruction set
    56.659 ms 55.211 ms : 32-bit BMI2 instruction set
    65.746 ms 68.178 ms : 32-bit AVX512 instruction set
    55.670 ms 268.382 ms : 64-bit BMI2 Instruction set
    55.339 ms 269.114 ms : 64-bit AVX512 Instruction set
    60.553 ms 291.424 ms : 32-bit BMI2 Instruction set
    59.651 ms 293.627 ms : 32-bit AVX512 Instruction set
```

Implements fully implements encoding for 2D and 3D cases.
Not particular optimized but passes all the tests.
Uses BMI2 placeholder implementation for decoding for verification

Current comparison against BMI2

```

CPU:       Topology: Quad Core model: Intel Core i7-1065G7 bits: 64 type: MT MCP L2 cache: 8192 KiB
           Speed: 834 MHz min/max: 400/3900 MHz Core speeds (MHz): 1: 969 2: 1018 3: 923 4: 1037 5: 708 6: 992 7: 613 8: 691

    02.050 ms 1.818 ms : 64-bit BMI2 instruction set
    04.213 ms 4.208 ms : 64-bit AVX512 instruction set
    1.835 ms 1.803 ms : 32-bit BMI2 instruction set
    4.222 ms 4.216 ms : 32-bit AVX512 instruction set
    2.021 ms 5.489 ms : 64-bit BMI2 Instruction set
    1.952 ms 5.462 ms : 64-bit AVX512 Instruction set
    01.964 ms 5.285 ms : 32-bit BMI2 Instruction set
    01.964 ms 5.517 ms : 32-bit AVX512 Instruction set
    14.823 ms 14.787 ms : 64-bit BMI2 instruction set
    33.683 ms 34.622 ms : 64-bit AVX512 instruction set
    14.487 ms 14.675 ms : 32-bit BMI2 instruction set
    33.629 ms 33.935 ms : 32-bit AVX512 instruction set
    15.448 ms 41.621 ms : 64-bit BMI2 Instruction set
    15.282 ms 43.666 ms : 64-bit AVX512 Instruction set
    15.635 ms 42.059 ms : 32-bit BMI2 Instruction set
    16.512 ms 44.433 ms : 32-bit AVX512 Instruction set
    137.746 ms 135.549 ms : 64-bit BMI2 instruction set
    314.947 ms 325.968 ms : 64-bit AVX512 instruction set
    136.730 ms 132.937 ms : 32-bit BMI2 instruction set
    315.256 ms 321.689 ms : 32-bit AVX512 instruction set
    141.504 ms 374.482 ms : 64-bit BMI2 Instruction set
    136.387 ms 381.539 ms : 64-bit AVX512 Instruction set
    153.533 ms 370.826 ms : 32-bit BMI2 Instruction set
    136.177 ms 377.794 ms : 32-bit AVX512 Instruction set
```

Removed unused math.h