Frobenius powers

Nobody has claimed this yet.

This operation on ideals is not well defined, i.e., equal ideals don't provide equal results.
We should make it mathematically sensible.

```
Ideal ^ Array := (I, e) -> (
R := ring I;
n := numgens R;
-- Error if input is not correct.
if any(e, i -> i < 0) then error "Expected nonnegative exponents.";
if #e != 1 and n != #e then error "Expected single integer array, or array with length equal to the number of variables.";
-- if only one element, then make vector the same as the length of
-- the number of variables with the same number in each entry
if #e == 1 then e = new Array from n:(e_0);
-- build a ring homomorphism that will perform this substitution for us
phi := map(R,R,matrix {apply(numgens R, i -> (R_i)^(e_i))});
-- apply the ring homomorphism and create the new ideal
ideal phi generators I
)
```

Start by locating the Ideal ^ Array operation described in the issue and review the proposed input checks and substitution behavior. Done means the operation is mathematically well defined: equal ideals produce equal results, while the stated exponent validation remains correct.

Written by the indexing model from the issue text.