Allow invoke to use with statements to specify combinational connections

[rachitnigam]

One pattern that showed up while writing #632 is that while invoking a Calyx component, it would be useful to also have some other combinational connections active. For example, if you have two memories a_mem and b_mem and want to pass a value at index idx to an invoked component, your two choices are:

1. Save the value a_mem[idx] in a register and pass the value of the register, or
2. Don't use the invoke statement at all.

A cleaner solution is to allow invoke statements to enable a combinational group that activates some assignments when the invoke group is running:

comb group mem_idx {
  a_mem.addr0 = idx.out;
  b_mem.addr0 = idx.out;
}

control {
  invoke mac(a = a_mem.read_data, b = b_mem.read_data) with mem_idx;
}

The compilation for such groups would be straightforward--inline all assignments in the comb group into the group generated to implement the invoke. Additionally, since this is specifying things using with, only combinational groups. I'm not sure if normal groups should be allowed anywhere where with is used.

[sampsyo]

Ah, this seems like a clear win! In retrospect, it seems like the natural way to pass "arguments" to "functions" using invoke. Good point!

[rachitnigam]

Requires #635 to be merged first.