[wpiutil] Add rotated_span

[PeterJohnson]

TODO:

• Change offset constructor parameters to int rotation instead
• Test rotations

[KangarooKoala]

A bit clunky, but if we want to make sure these are invalid, we could define a concept that checks if a call with a parameter is valid (using a requires expression) and then use static asserts:

template <auto Param, auto Func>
concept ValidParamFor = requires {
  Func(Param);
};

static_assert(!ValidParamFor<csarr_values, unsized_func>);

Note that we do need to define the concept, since "If a substitution failure would occur in a requires-expression for every possible template argument, the program is ill-formed, no diagnostic required." (requires expression)