quite a big intrusive change in auto-assignment

quite a big intrusive change in auto-assignment
variables that do not exist get auto-created based on the bit-width
at which they are first encountered

    prod[0:31]

creates a variable with

    prod = concat(0, repeat=32)

however this needs to be more complicated rather than just assume
it is a pair of constants

expressions can now be

     prod[0:XLEN-1]

which gets an ast.BinOp expression created on the RHS.

therefore allow the assignment "var = concat(...., repeat=xxxx)"
to accept computed expressions by returning an ast.BinOp(UPPER, "-", LOWER)
so that the resultant python code performs the subtract calculation