Compiletime scripts

[Adrodoc]

Expanding on the idea of #25 it would be cool to enable embedded scripts that generate parts of the MPL program. This would require MPL to package a full language Interpreter.
Because MPL is written in Java, embedded groovy scripts are a good option.
An embedded script could look like this:

/say normal MPL code
compiletime {
  for (i in 1..10) {
    println "/generated line ${i}"
  }
}

Instead of the keyword compiletime we could use embedded or groovy.
For security reasons it is important to restrict embedded scripts from accessing the file system or internet.

[Adrodoc]

Alternative syntax used in d490da5:

compiletime {
  for (i in 1..10) {
    mpl << "/generated line ${i}"
  }
}

[BrainStone]

I would suggest using lua instead.

First of all the syntax is quite easy and you still can to pretty much everything you can with groovy and second the library is tiny compared with the groovy library.