refactor: Improve AVX, make detection a little nicer

[braindigitalis]

• split out isa into four separate files, rather than one long snakepit of #ifdef
• pass flags properly to caller, and have the caller set the compile flags and define
• set the define as a fixed name with a value 0, 1, 2 or 512, instead of four different defines
• Test compiled successfully on my server (AVX2) and my raspberry pi 4b (AVX0 fallback) More tests especially on windows welcome

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