Speedup struct decoding.

```
benchmark                                  old ns/op     new ns/op     delta
BenchmarkFormatQWithoutArgs                1403          1356          -3.35%
BenchmarkFormatQWithArgs                   6300          6220          -1.27%
BenchmarkFormatQWithStructFields           7343          6829          -7.00%
BenchmarkFormatQWithStructMethods          10170         9665          -4.97%
BenchmarkQuery                             4022153       3026034       -24.77%
BenchmarkQueryStdlibPq                     7897578       7719953       -2.25%
BenchmarkQueryRow                          134965        121860        -9.71%
BenchmarkQueryRowStmt                      129250        121326        -6.13%
BenchmarkQueryRowLoadInto                  123896        118824        -4.09%
BenchmarkQueryRowStmtLoadInto              105662        110593        +4.67%
BenchmarkQueryRowStdlibPq                  364151        344920        -5.28%
BenchmarkQueryRowWithoutParamsStdlibPq     155965        172191        +10.40%
BenchmarkQueryRowStdlibMySQL               117470        116288        -1.01%
BenchmarkQueryRowStmtStdlibPq              193287        204958        +6.04%
BenchmarkExec                              147404        151520        +2.79%
BenchmarkExecWithError                     167628        178578        +6.53%
BenchmarkExecStmt                          134515        146804        +9.14%
BenchmarkExecStmtStdlibPq                  222488        208082        -6.47%
```