Add algebraic operations between dates

[ecyshor]

We have a number of operations which can be executed between dates and numbers, but no support to subtract two dates. This should produce a column of int as result.

[erdebee]

I do not think this is true, there are magnets for DateTime to AddSubtractable.

select(const(new DateTime()) - 1)

compiles and produces a valid qry

[ecyshor]

That is true, but there is no support to subtract two columns which represent a date (this would return the number of days between the dates). Should be pretty straightforwards to implement the magnet for localdate, localdate, int