fix: pow and daysBetween are not commutative

[skymanbp]

normalize reorders the operands of commutative operations by expression key. pow and daysBetween were flagged commutative, so pow (x + 1) 2 could run as 2 ^ (x + 1), and the sign of daysBetween followed the column names. Both are now flagged not commutative. The daysBetween example in the docs relied on the swap and now passes the deadline first.

Before, with x = [2, 4]:

ghci> D.columnAsList @Int (F.col "sq") (D.derive "sq" (F.pow (F.col @Int "x" + F.lit 1) 2) df)
[8,32]

After:

[9,25]

With start = 2024-03-01 and end = 2024-03-10, daysBetween start end gave 9 and now gives -9. daysBetween end start gives 9 in both.

[mchav]

This is a really sad kind of Boolean blindness. Maybe we should make a small type that encodes this and makes it easier to reason locally about each of these definitions.

[mchav]

Can you file that as a follow up issue?

[skymanbp]

Can you file that as a follow up issue?

sure, no problem.

[skymanbp]

Filed as #234.