The pass-by-value catastrophe

[akkuankka]

Currently everything in the computation side of the code (i.e. not the lexer) is pass-by value (i.e. fn foo(self)). I think this is mostly OK, because stuff doesn't need to be used once it's combined into another value e.g. let c = a + b once that's been done it really doesn't matter what a is, it can go out of scope, but for things like DivisibleBy where you are just gaining some knowledge about a number and then using that to decide what to do with the number, pass-by-reference (i.e. fn foo(&self)) should be used.