feature: All structs are ported to use smol_str::SmolStr for owned variant…

[damooo]

It addresses #21

It uses smol_str::SmolStr as string invariant for owned segment, and &str for borrowed.

smol_str is dependable, as it is maintained by rust_analyzer team, and used in it extensively.

[damooo]

@sgodwincs , In this, all structs that use Cow<'s, str> are ported to use SmolStrCow<'s>.

With this, in most cases, owned uri is as cheap as borrowed one, and can be passed around, and included in structs with out heavy lifetime management as we can do with url::Url, http::Uri, etc.