Support multiple instances of the VM in a process

[DemiMarie]

This is a feature request to support multiple instances of Femtolisp in a process. This is useful for when Femtolisp is embedded in a C or C++ application.

[JeffBezanson]

Luckily, this has been implemented in the fork of femtolisp embedded in julia (https://github.com/JuliaLang/julia/tree/master/src/flisp) to support multi-threading. So it should not be too hard to port over. Please feel free to tackle this if you'd like, and open a pull request.