
I would like you to implement a module named TopModule with the following
interface. All input and output ports are one bit unless otherwise
specified.

 - input  a   (8 bits)
 - input  b   (8 bits)
 - input  c   (8 bits)
 - input  d   (8 bits)
 - output min (8 bits)

The module should find the minimum of the four input values. Unsigned
numbers can be compared with standard comparison operators (a < b).

