perf: change ports type to Option<NonZeroU16>

[DonIsaac]

Proposal implementation for #930

I've left the public API unchanged; all public port getter/setters still deal with u16s.

[valenting]

Doesn't this make it impossible to parse http://example.org:0/ ?
That's still a valid URL according to the URL standard.

[DonIsaac]

Yes, it is a valid port. It appears to be used for some use cases in the wild. I'm wondering if/how we could get this memory layout improvement without impacting users that need port 0. Maybe a feature flag?

[valenting]

I don't think this is possible for port, but I encourage you to explore options for

    query_start: Option<u32>,    // Before '?', unlike Position::QueryStart
    fragment_start: Option<u32>, // Before '#', unlike Position::FragmentStart

If I'm not mistaken, it should be impossible for them to start at 0.