add unmanaged-kcp transport

[Molth]

No description provided.

[SoftwareGuy]

Got any performance benchmarks to back up these claims? That'll boost possibility of getting it merged into Mirror mainline.

[miwarnec]

@Molth is this still worth it over threaded C# kcp? and have you compared this against IL2CPP compiled kcp?
perf. should be similar with IL2CPP.

there are a few reasons why we chose pure C# for kcp:

• able to inspect and debug all the code easily
• we fixed a few bugs in kcp that skywind never addressed
• C# version runs on all platforms. native we'll have to compile on windows/mac/linux/mobile/consoles/etc.

[James-Frowen]

after a quick look, I would guess the increase in performance is from using memcpy instead of MemoryStream. I would assume there could be similar gain using byte[] and Buffer.BlockCopy

the unsafe/unmanaged code itself shouldn't give any major performance increase over managed c#, because all that will do is skip safety bounds checks on arrays.