[byteTree] Write fixed sized data using an optimized form

[ahoppen]

If we know the size of a type at compile time (like we do for all the integer types), it is cheaper to assign the data buffer directly instead of using a memcpy.

[ahoppen]

@swift-ci Please smoke test

[nkcsgexi]

Out of curiosity, what difference can this optimization make, i.e. *(T *)(Data.data() + Offset) = Value; vs. ::memcpy(Data.data() + Offset, Buffer.data(), Buffer.size());?

[ahoppen]

I would need to test again for exact numbers, but if I remember correct it improved serialisation performance by around 5%

[nkcsgexi]

ah, OK. Then i think the change is worth it. 👍

[ahoppen]

@swift-ci Please smoke test