josephwright
Always evaluate \raiseto and \tothe input
Comes up from https://tex.stackexchange.com/questions/693582/bug-in-siunitx/693608. Currently, \tothe assumes that the input is a simple value. This means if no further maths takes place, it is simply printed even if a complex value. However, adding \per forces evaluation and a potentially a confusing error. There should be finer control of this: likely such a change would need to go with a way to convert decimals to fractions.
There is another use case where mathmatical expressions in \tothe is desired. In chemical kinetics, the unit of rate constant is usually written as \unit{\mole\tothe{1 - \alpha}\liter\tothe{\alpha - 1}\per\second}, where \alpha is the overall reaction order. When we don't know the reaction order in advance (for example, when we are faced with a completely new reaction and we want to determine its kinetic properties by multivariate fitting), such expressions will be useful. However, if we turn off parse-units, \per will be printed as a slash whatever per-mode is, which may cause inconsistency.
I think we also need https://github.com/josephwright/siunitx/issues/683 sorting - so bigint first!