Add SIMD support for imagehash

[evanoberholster]

Use of float32 array instead of float64 array. Uses less memory and able to apply faster SIMD functions.
YCbCr conversion to float32 is not exact and is approximate

name                      old time/op    new time/op    delta
PHash64/Fast32-12           37.3µs ± 1%     6.2µs ± 1%  -83.33%  (p=0.000 n=17+16)
PHash64/Fast-Parallel-12    6.25µs ± 1%    0.90µs ± 0%  -85.61%  (p=0.000 n=19+19)

name                      old alloc/op   new alloc/op   delta
PHash64/Fast32-12            0.00B          0.00B          ~     (all equal)
PHash64/Fast-Parallel-12     0.00B          0.00B          ~     (all equal)

name                      old allocs/op  new allocs/op  delta
PHash64/Fast32-12             0.00           0.00          ~     (all equal)
PHash64/Fast-Parallel-12      0.00           0.00          ~     (all equal)

[evanoberholster]

Added support for DCTHash256.

name                       old time/op    new time/op    delta
PHash256/Fast-12              739µs ± 2%     124µs ± 3%     -83.21%  (p=0.000 n=20+20)
PHash256/Fast-Parallel-12     120µs ± 1%      16µs ± 1%     -86.29%  (p=0.000 n=20+20)

name                       old alloc/op   new alloc/op   delta
PHash256/Fast-12             3.89B ±234%  1033.00B ± 2%  +26462.86%  (p=0.000 n=18+20)
PHash256/Fast-Parallel-12    2.19B ±129%  1040.10B ± 1%  +47447.43%  (p=0.000 n=16+20)

name                       old allocs/op  new allocs/op  delta
PHash256/Fast-12               0.00           1.00 ± 0%       +Inf%  (p=0.000 n=20+20)
PHash256/Fast-Parallel-12      0.00           1.00 ± 0%       +Inf%  (p=0.000 n=20+20)

TODO:

• Update and improve tests

[coveralls]

Pull Request Test Coverage Report for Build 4418030528

• 46 of 52 (88.46%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+2.9%) to 33.826%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
imagehash/imagehash32.go	46	52	88.46%

Totals
Change from base Build 4275565407:	2.9%
Covered Lines:	1992
Relevant Lines:	5889

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💛 - Coveralls