Kernel of ring maps in mixed characteristic

[mahrud]

Here's a simple computation that does not currently work:

S = ZZ[x_1,x_2,x_3]
R = ZZ[e_1, e_2, e_3, Degrees =>{1,2,3}]

f = map(S, R, {x_1 + x_2 + x_3, x_1*x_2 + x_1*x_3 + x_2*x_3, x_1*x_2*x_3})
assert isHomogeneous f
kernel f -- stdio:6:1:(3): error: kernel RingMap: not implemented yet

One fix would be to generalize the existing strategy that uses graphIdeal, which also doesn't currently work:

graphIdeal f -- stdio:7:1:(3): error: expected an affine ring

But this should be easy to fix, since the graph isn't defined differently over integers.

Reported by @sashapevzner