Add support for implements clause

[ricochet]

DO NOT MERGE until wasm-tools release with
bytecodealliance/wasm-tools#2453

The component model implements clause allows a world to import
or export the same interface multiple times under
different plain names.

This threads the new implements:Option<InterfaceId> field from
wit-parser through the WorldGenerator trait
and all backends (Rust, C, C++, C#, Go, MoonBit, Markdown).

A shared wasm_import_module_name helper
produces the [implements=<I>]label binary encoding.

Key per-backend concerns handled:

• Type dedup: types are generated once per InterfaceId,
  not per label
• Wasm names: all wasm-level import/export names use the
  implements encoding when applicable
• Distinct bindings: each label gets its own
  module/namespace with independent function bindings

[alexcrichton]

The guest side of things I think is going to be tricky in its own right compared to the host side of things. Notably if an interface is imported twice this'll need to generate two different types for all types contained within the interface. Without that it's not a reflection of the original WIT itself, so this means that (at least from my understanding at a skim) the duplicate-related handling will need to be removed.

To handle this though what I'd recommend is updating the Resolve::generate_nominal_type_ids method. That would transform imports of the same interface multiple times into imports of distinct interfaces which will make it much easier to support this in bindings generators (different InterfaceIds and TypeIds for different interfaces/types). The logic of generate_nominal_type_ids will be a bit tricky, but I think that's the best foundational primitive to use here.