Special packing for complex, specialize packing for avx2

[bluss]

Complex (cgemm, zgemm):

Use a different pack layout for complex micorkernels which puts real and
imag parts in separate rows. This enables much better autovectorization for
the fallback kernels.

Also enable an Avx2 + Fma autovectorized kernel.

Performance improvements (all kernels autovectorized for cgemm, zgemm
at this time)

• AArch64 NEON (apple m1): cgemm: +60%, zgemm +10%
• Fma + Avx (Intel Tiger lake) cgemm: +143%
• Fma + Avx2 (Intel Tiger lake) cgemm: +395% (new), zgemm: +77% (new)

Float (sgemm, dgemm):

When the kernels can now select their own packing functions, instantiate
an avx2 version of the general packing function for sgemm and dgemm.

Packing performance matters most for small matrix multiplications, for
bigger sizes it is a vanishingly small part of runtime.

• Avx2 (Intel Tiger Lake): sgemm improves 6-15%, dgemm improves 0-8%
  depending on input layouts. Tested on M, K, N = 32, i.e a small matrix.

[bluss]

avx2 packing bench

 name                        nobeta-avx-before1 ns/iter  nobeta-avx-after1 ns/iter  diff ns/iter   diff %  speedup 
 layout_f32_032::nobeta_ccc  1,257                       1,143                              -114   -9.07%   x 1.10 
 layout_f32_032::nobeta_ccf  1,251                       1,140                              -111   -8.87%   x 1.10 
 layout_f32_032::nobeta_cfc  1,445                       1,259                              -186  -12.87%   x 1.15 
 layout_f32_032::nobeta_cff  1,441                       1,255                              -186  -12.91%   x 1.15 
 layout_f32_032::nobeta_fcc  1,080                       1,020                               -60   -5.56%   x 1.06 
 layout_f32_032::nobeta_fcf  1,074                       1,018                               -56   -5.21%   x 1.06 
 layout_f32_032::nobeta_ffc  1,287                       1,147                              -140  -10.88%   x 1.12 
 layout_f32_032::nobeta_fff  1,280                       1,142                              -138  -10.78%   x 1.12 
 layout_f64_032::nobeta_ccc  1,761                       1,783                                22    1.25%   x 0.99 
 layout_f64_032::nobeta_ccf  1,760                       1,776                                16    0.91%   x 0.99 
 layout_f64_032::nobeta_cfc  1,920                       1,839                               -81   -4.22%   x 1.04 
 layout_f64_032::nobeta_cff  1,914                       1,830                               -84   -4.39%   x 1.05 
 layout_f64_032::nobeta_fcc  1,636                       1,581                               -55   -3.36%   x 1.03 
 layout_f64_032::nobeta_fcf  1,632                       1,572                               -60   -3.68%   x 1.04 
 layout_f64_032::nobeta_ffc  1,766                       1,634                              -132   -7.47%   x 1.08 
 layout_f64_032::nobeta_fff  1,760                       1,627                              -133   -7.56%   x 1.08