[VL] Support integers in functions

Here's the error message:

```
VL-BAD-FUNCTION-VARDECL (fatal, from vl-fun-vardecllist-types-okp)
In Function foo, variable idx is not a simple register. Other types of
variables are not yet supported.
```

Some sample code to generate the error:

```
function [15:0] foo;
input [3:0] addr;
integer idx;
begin
// need some other stuff to set other values, maybe
foo[idx] = (idx == addr[3:0]);
end
endfunction
```