Add implements clause support to wac

[ricochet]

Adds support for composing components that use the component model (implements "I") clause (component-model#613), which lets a component import or export the same interface more than once under different plain-names.

  world consumer {
    import primary: wasi:keyvalue/store;
    import backup:  wasi:keyvalue/store;
  }

In the binary this encodes each extern with a plain-name label and an (implements "…") directive carrying the interface it implements:

(component
  (import "primary" (implements "wasi:keyvalue/store") (instance ...))
  (import "secondary" (implements "wasi:keyvalue/store") (instance ...))
)

This change makes it so that each labeled extern is preserved as its own instance and round-trips the implements directive.

Bump to latest wasm-tools which has implements support. Part of this was also adding ComponentItem to TypeConverter::entity, and for an instance extern, resolves the interface id from the implements metadata when present.

[fibonacci1729]

@ricochet thanks for driving this! Ack'ing your review request -- i'll start digging into this over the next few days / week.

[fibonacci1729]

This looks great!

I think it would be great to add a test that explicitly exercises the spec's more subtle transitive use case where two labeled imports of an interface should share a separately imported types interface resource instead of duplicating it. I don't believe the code mishandles that case, but it is the main scenario I'd like to see covered because the spec discussion calls it out as important.