Don't use dynamic cast if static_cast is enough

In case when TTsonStructBase inherited non-virtually (which appears,
by my measurements, in 2/3 cases) static_cast to derived class
seems to work fine.

This gives improvement from:
```
BenchmarkEncode<TFlatStructLite>        4458 ns         4459 ns       153900 items_per_second=224.285k/s
BenchmarkEncode<TFlatStructHeavy>       4392 ns         4392 ns       156537 items_per_second=227.663k/s
BenchmarkEncode<TDeepStructLite>       41765 ns        41761 ns        16616 items_per_second=23.9456k/s
BenchmarkEncode<TDeepStructHeavy>       9488 ns         9487 ns        73414 items_per_second=105.405k/s
BenchmarkDecode<TFlatStructLite>       24437 ns        24433 ns        29036 items_per_second=40.9277k/s
BenchmarkDecode<TFlatStructHeavy>      24807 ns        24804 ns        29581 items_per_second=40.3154k/s
BenchmarkDecode<TDeepStructLite>       88384 ns        88365 ns         7578 items_per_second=11.3167k/s
BenchmarkDecode<TDeepStructHeavy>      42440 ns        42432 ns        16215 items_per_second=23.5672k/s
```
To
```
BenchmarkEncode<TFlatStructLite>        2511 ns         2511 ns       255493 items_per_second=398.19k/s
BenchmarkEncode<TFlatStructHeavy>       2614 ns         2614 ns       275356 items_per_second=382.587k/s
BenchmarkEncode<TDeepStructLite>       19911 ns        19911 ns        33934 items_per_second=50.2243k/s
BenchmarkEncode<TDeepStructHeavy>       5578 ns         5578 ns       127230 items_per_second=179.292k/s
BenchmarkDecode<TFlatStructLite>       20227 ns        20224 ns        36001 items_per_second=49.4471k/s
BenchmarkDecode<TFlatStructHeavy>      19402 ns        19398 ns        35820 items_per_second=51.5512k/s
BenchmarkDecode<TDeepStructLite>       51096 ns        51091 ns        10000 items_per_second=19.5728k/s
BenchmarkDecode<TDeepStructHeavy>      33854 ns        33854 ns        22125 items_per_second=29.5388k/s
```
Part 5 of https://nda.ya.ru/t/E9Ji-1bm7Gwv6c
commit_hash:d4d35a26a8872ed5d905c6ba878ad50b93a62d79

0 files changed, 0 insertions, 0 deletions