Support expression for index

[jyx0615]

It would be helpful to add support for expressions in array indexing.

For example, in a nested generate_seq, I would like to index arrays using arithmetic expressions such as 8*i + j:

generate_seq(i : 0, 7, 1) {
    generate_seq(j : 0, 7, 1) {
        let aw_req = recv demux_re[8*i + j].mst_aw_req >>
        send mux_re[8*j + i].aw_req(aw_req) >>
    }
}