Add avx2 optimization for encoder and decoder

[tszumski]

1. Fix bitstream mismatch between ojph_encode_codeblock() and ojph_encode_codeblock_avx512()
2. ojph_encode_codeblock_avx2() implementation
3. ojph_decode_codeblock_avx2() implementation

Based on my tests:
Encoder: performance gain ~37% vs C-code, (~7.5% performance drop vs AVX512-code​)
Decoder: performance gain ~7%

[aous72]

Hi Tomasz,

Thank you for this highly-needed PR.
I will try to merge it this week; I just need to run a few tests/checks.

Kind regards,
Aous.