Kill `UniFfiTag`

[mhammond]

Works perfectly - until we get to custom types :(

error[E0117]: only traits defined in the current crate can be implemented for types defined outside of the crate
  --> /Users/skip/src/moz/uniffi-rs/target/debug/build/uniffi-example-custom-types-d2c6ba7186877117/out/custom-types.uniffi.rs:73:1
   |
73 | ::uniffi::custom_type!(r#Url, String);
   | ^^^^^^^^^^^^^^^^^^^^^^^-----
   | |                      |
   | |                      `Url` is not defined in the current crate
   | impl doesn't use only types from inside the current crate
   |
   = note: define and implement a trait or new type instead
   = note: this error originates in the macro `::uniffi::custom_type` (in Nightly builds, run with -Z macro-backtrace for more info)