Support Arc<_> and impl blocks in proc-macro frontend

[jplatte]

This is incomplete, but I wanted to post a PR anyways so others can have a look at the progress.

Left to do:

• To avoid problems from the same name meaning different things across modules that #[uniffi::export] is used in,
  • create a new macro that's used at the crate root where users have to list the named types they want to use in FFI, sth. like uniffi::types! { Foo, some_place::Bar }, which will just expand to mod uniffi_types { use {Foo, some_place::Bar}; }
  • assert that the local Foo and crate::uniffi_types::Foo are the same type in the generated code, somehow (shouldn't be very hard)
• uniffi_bindgen: Read method metadata files and add them to ComponentInterface

Part of #1257.

[jplatte]

I think I'll actually skip the types! macro, it doesn't really do anything better than what it would expand to. That is, I think we should instead require people to define a uniffi_types module using the regular syntax.

[jplatte]

Ready for review! 🎉

[jplatte]

Ah, it looks like we need to generate extra scaffolding code for destructors. I hope the force-push I just did works around that, so this PR can be merged as-is instead of growing more in terms of complexity and code size.