Nimscripter - Easy Nimscript-Nim interop

[Araq]

For my toy benchmarks (recursive fib, iterative fac) Nim's VM is slower than 
Python by factors of 3 to 5 (ignoring the slower startup). That's **much** 
worse than anticipated. There is probably some low-hanging fruit here, I 
remember benchmarking it years ago and back then it was faster than Python. ;-)