Custom types can cause multiple imports

[bendk]

For many bindings, our custom type configuration can specify imports to add in order to support the custom type. The bindings code de-dupes these to avoid duplicate imports, however it doesn't take into account imports that are always present in the generated code. If a user specified one of these modules it would result in a duplicate import, which may be messy or may even prevent compilation. If a user doesn't specify the module because of this, then their bindings may be broken by a UniFFI upgrade that removes the import.

I think the solution is to put all imports from all sources into a BTreeSet to de-duplicate them.