Change representation of instruction set extensions

[serge-sans-paille]

Both Intel and ARM have instruction set extensions : fma for intel, i8mm for arm, not counting the avx512 extensions too.
We currently represent these extensions as a parametric type parametrized by the instruction set it extends, eg fma<avx>.

It would be nice to explore a more intuitive representation, maybe avx<fma, ...> ?

[serge-sans-paille]

extra thoughts: this would make it possible to catch avx2 extensions brought by avx512, and that's quite nice!