Wasm3 (nim library) how to run c functions

[ElegantBeef]

Using non-sized strings across the barrier will always be a pain since you 
manually have to copy each byte. I did push out a new version with a test 
example and introducing `copyFromMem` to make it easier to copy byte buffers 
from VM to Nim. 
<https://github.com/beef331/wasm3/commit/15d8eb143afb665ab4ea11205f62f7c25aec35e7>