T2: Use 8b packed output of the T2 encoder

- modify retrieve_ldpc_enc_op() to store encoded results in a packed format
- add timing measurements for T2 encoder
- add simde implementation for bit reversal
  - Introduce new bit reversal implementation using simde_mm_gf2p8affine_epi64_epi8
    instruction, based on https://wunkolo.github.io/post/2020/11/gf2p8affineqb-bit-reversal/.
    This implementation requires GFNI and AVX512VL support on the target machine.

common/utils/nr/nr_common.c

@@ -37,6 +37,7 @@
 #include "nr_common.h"
 #include <limits.h>
 #include <math.h>
+#include <simde/x86/gfni.h>
 
 #define C_SRS_NUMBER (64)
 #define B_SRS_NUMBER (4)
@@ -131,8 +132,24 @@ void reverse_bits_u8(uint8_t const* in, size_t sz, uint8_t* out)
   DevAssert(in != NULL);
   DevAssert(out != NULL);
 
+// Bit reversal implementation based on https://wunkolo.github.io/post/2020/11/gf2p8affineqb-bit-reversal/
+#if defined(__GFNI__) && defined(__AVX512VL__)
+  int simde_sz = 16;
+  int i = 0;
+  int simde_bound = sz - simde_sz;
+  for (; i <= simde_bound; i += simde_sz) {
+    __m128i input = simde_mm_loadu_si128((__m128i *)&in[i]);
+    __m128i reversed = simde_mm_gf2p8affine_epi64_epi8(input, simde_mm_set1_epi64x(0x8040201008040201), 0);
+    simde_mm_storeu_si128((__m128i *)&out[i], reversed);
+  }
+
+  for (; i < sz; ++i) {
+    out[i] = bit_reverse_table_256[in[i]];
+  }
+#else
   for(size_t i = 0; i < sz; ++i)
     out[i] = bit_reverse_table_256[in[i]];
+#endif
 }
 
 // Reverse bits implementation based on http://graphics.stanford.edu/~seander/bithacks.html