I wonder though, I know a lot of tricks for performance are used, such as in python API, judicious use of inlining, putting some common functions in headers files etc. Depending on how far down the rabbit hole you want to go, could this library not also include code generation functionality for JIT's? Or even at compile time, is there anything stopping a library from generating code that could be included in your VM runtime as just an opaque function that can be called?
At that point you're effectively creating your own VM, and then you may ask why doesn't someone simply use the JVM or the CLR. Once you reach that degree of complexity, there are simply too many tradeoffs.
I certainly won't stop you from trying to develop such a thing, could be a nice way to get languages bootstrapped faster. Just keep in mind JIT compilation is forbidden in iOS, so e.g. C# needs an entirely different compilation strategy for that platform.
