Don't construct twice yson struct members of yson struct lite type

There is a serious problem of all yson structs: their members are
constructed twice when the yson struct is constructing: the first
time implicitly as common members, and the second time explicitly
right in yson struct constructor's body.

So if any yson struct A has a member of yson struct lite type B,
then per one A's construction the B will be constructed twice. If
in turn B has a member of yson struct lite type C then it will be
contructed two time per B's contruction and four times per A's
construction. The next level D will be constructed 8 times etc,
generally giving compexity of 2^N for construction of yson struct
lite chain of length N.

This patch solves the most common case when yson struct lite member
is declared as .Default() or .Otional(). In this case ther's no
need to construct the member the second time.

This gives improvement from:
```
BenchmarkDecode<TDeepStructLite>       88384 ns        88365 ns         7578 items_per_second=11.3167k/s
```
To:
```
BenchmarkDecode<TDeepStructLite>       35361 ns        35359 ns        19786 items_per_second=28.2814k/s
```
Part 3 of https://nda.ya.ru/t/E9Ji-1bm7Gwv6c
commit_hash:b8a3db2cdc4393bae38b7310047cc3fcd9b9a15c

0 files changed, 0 insertions, 0 deletions