Implement functions for `EntityScript`

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Functions would be subroutines that can be added to a script via merge composition.

The syntax would look something like this:

function f(x, y:int, z:string)
    return ((x + y):string + ", " + z)
end

[ImmutableOctet]

Leaving this on hold, since we're going to be writing most logic with the C++ script interface anyway.