avoid generating duplicate stream and future constructors

[dicej]

Previously, we used the payload type to avoid generating duplicates, but that doesn't work reliably because wit-parser does not necessarily deduplicate types involving aliases, such as the following example:

interface types {
  variant foo {
    bar,
    baz
  }
}

interface use-types-a {
  use types.{foo};

  test: func() -> future<result<foo>>;
}

interface use-types-b {
  use types.{foo};

  test: func() -> future<result<foo>>;
}

When targetting a world which imports and/or exports both use-types-a and use-types-b, we'll end up with distinct type IDs for result<foo>.

This commit switches to using wit-bindgen-core to determine the "canonical" type of the payload and then use that as the deduplication key. This unblocks using wasi:cli@0.3.0-rc-2026-02-09, which has a similar pattern to the above WIT example.

Note that this is awkward to test given our current test infra, but #205 will cover it once it's merged.

[dicej]

Actually, I just realized wit_bindgen_core supports a better way to do this; going to use that instead.