Conditional compilation

[lightclient]

Sometimes it would be nice to compile things in based on a flag. For example, maybe you want overflow detection to be optional. Here a couple potential approaches:

%import_if(foo, "safemath.etk")
%import_if(!foo, "unsafemath.etk")

or

#if safe_overflow
...
#endif

I think I prefer the latter because I expect inline usage to be common for a couple quick ops.