avoid genericAssign for beacon node types (#1166)
* avoid genericAssign for beacon node types
ok, I got fed up of this function messing up cpu measurements - it's so
ridiculously slow, it's sad.
before, while syncing:
```
40,65% beacon_node_shared_witti_0 [.]
genericAssignAux__U5DxFPRpHCCZDKWQzM9adaw
9,02% libc-2.31.so [.] __memmove_avx_unaligned_erms
7,07% beacon_node_shared_witti_0 [.] BIG_384_58_monty
5,19% beacon_node_shared_witti_0 [.] BIG_384_58_mul
2,72% beacon_node_shared_witti_0 [.] memcpy@plt
1,18% [kernel] [k] rb_next
1,17% beacon_node_shared_witti_0 [.] genericReset
1,06% [kernel] [k] map_private_extent_buffer
```
after:
```
24,88% beacon_node_shared_witti_0 [.] BIG_384_58_monty
20,29% beacon_node_shared_witti_0 [.] BIG_384_58_mul
3,15% beacon_node_shared_witti_0 [.] BIG_384_58_norm
2,93% beacon_node_shared_witti_0 [.] BIG_384_58_add
2,55% beacon_node_shared_witti_0 [.] BIG_384_58_sqr
1,64% beacon_node_shared_witti_0 [.] BIG_384_58_mod
1,63% beacon_node_shared_witti_0 [.]
sha256Transform__BJNBQtWr9bJwzqbyfKXd38Q
1,48% beacon_node_shared_witti_0 [.] FP_BLS381_add
1,39% beacon_node_shared_witti_0 [.] BIG_384_58_sub
1,33% beacon_node_shared_witti_0 [.] BIG_384_58_dnorm
1,14% beacon_node_shared_witti_0 [.] FP2_BLS381_mul
1,05% beacon_node_shared_witti_0 [.] BIG_384_58_cmove
1,05% beacon_node_shared_witti_0 [.]
get_shuffled_seq__4uncAHNsSG3Pndo5H11U9aQ
```
* better field iteration
78b767f645