LEWGI Belfast: more minor issues

[eliaskosunen]

• [ ] constexpr default constructor
• [ ] Audit noexceptness, keeping in mind the Lakos rule
• [ ] process::native_exit_code return type should be implementation-defined
• [x] Remove process(pid) constructor
• [ ] Error handling exploration paper (I have a number, D1977)

[eliaskosunen]

The fourth bullet is a duplicate of #28, despite the issue title

[JeffGarland]

Do you want to handle the drafting of 1977 in this repo or somewhere else?