Rust default array fix

[jergussuja]

Generated object api struct implement Default trait by derive macro, but array implement Default trait only for size up to 32 (source).

Main contribution goes to @DolajoCZ, I just verified C++ code changes!

Fixes #8436.

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[TethysSvensson]

This fails to generate valid code for the following schema:

struct MyStruct2 {
  x: float;
}

struct MyStruct {
  x: [MyStruct2: 10];
}

table Root {
  x: MyStruct;
}

[jergussuja]

This fails to generate valid code for the following schema:
...

Thanks for your check! Should be fixed.