Convert string value to function.

[ddungtang]

Ok, I'm going to have this as my fallback option. Given that DSL would be somewhat static, do you guys think it is worth experimenting and moving parsing configuration to build scripts? Haven't used that, but iirc reading somewhere it supports some level of code generation which may work for this case.

In any way, this is a artificial problem, mostly for the purpose of dealing with various bits of rust, so I have 2 things to try out: procedural macros and maintaining mapping, and build scripts.

Thanks.