Add paternalistic default to option parameters to None if the user didn't specify one

[calippo]

requirements

• Add paternalistic default to option parameters to None if the user didn't specify one

specs

given:

wa: Option[Int]

convert it to:

wa: Option[Int] = None

misc

We spent some time trying to solve https://github.com/buildo/wiro/issues/32 while it was not a real bug. That's why we decided to go with a paternalistic default.