Add support for non-literal array lengths

[henridf]

The following example fails due to the array length not being a *ast.BasicLit.

package badone

const ArrayLength = 1

type S struct {
    Arr [ArrayLength]byte `ssz-size:"10"`
}

It would be handy to be able to support such cases (this is coming up when sszgen-annotating geth's Header struct).

[ferranbt]

I will try to get something rolling on this for this week.