islinear in src/csimp.lisp

This is OK:

(%i2) demoivre(exp(%i* a[x]));
(%o2) %i*sin(a[x])+cos(a[x])

This isn't:

(%i3) demoivre(exp(%i* a[%i]));
(%o3) %e^(%i*a[%i])

This happens because the function islinear determines
that %i * a[%i] isn't linear in %i.