encoding/json: realign struct encodeState, structFields, and field

This reduce the struct encodeState size from 56 to 16 bytes (-42),
struct structFields size from 40 to 24 bytes (-16), and struct field size
from 136 to 128 bytes (-8).

Benchmark memory profiling before and after,

File: json.test
Type: alloc_space
Time: Jan 25, 2024 at 4:51am (WIB)
Showing nodes accounting for -4.01GB, 4.66% of 85.95GB total
Dropped 64 nodes (cum <= 0.43GB)
      flat  flat%   sum%        cum   cum%
   -4.25GB  4.94%  4.94%    -4.01GB  4.67%  encoding/json.Marshal
    0.19GB  0.22%  4.73%     0.24GB  0.28%  encoding/json.mapEncoder.encode
    0.05GB 0.057%  4.67%     0.05GB 0.057%  reflect.copyVal
    0.04GB 0.049%  4.62%     0.04GB 0.049%  bytes.growSlice
   -0.04GB 0.045%  4.66%    -0.04GB 0.045%  encoding/json.RawMessage.MarshalJSON
         0     0%  4.66%     0.04GB 0.046%  bytes.(*Buffer).WriteString
         0     0%  4.66%     0.04GB 0.049%  bytes.(*Buffer).grow
         0     0%  4.66%    -0.04GB 0.045%  encoding/json.(*Encoder).Encode
         0     0%  4.66%     0.20GB  0.23%  encoding/json.(*encodeState).marshal
         0     0%  4.66%     0.20GB  0.23%  encoding/json.(*encodeState).reflectValue

0 files changed, 0 insertions, 0 deletions