[wasm-metadata] re-use metadata fields between modules and components

[yoshuawuyts]

Currently the Metadata enum in wasm-metadata has two variants: Module and Component. Both inline all of their fields, with the only difference between the two being that Component has an additional children field. This leads to a fair number of duplicate code paths.

This PR adds a new enum Payload and changes Metadata from an enum to a struct, which is shared between both variants of Payload. This makes it easier to work with, reason about, and add new metadata fields to wasm-metadata. Thanks!

Summary

// Before: single enum with duplicate fields
enum Metadata {
     Component {
         name: Option<String>,
         producers: Option<Producers>,
         author: Option<Author>,
         description: Option<Description>,
         licenses: Option<Licenses>,
         source: Option<Source>,
         children: Vec<Box<Metadata>>,
         range: Range<usize>,
     },
     Module {
         name: Option<String>,
         producers: Option<Producers>,
         author: Option<Author>,
         description: Option<Description>,
         licenses: Option<Licenses>,
         source: Option<Source>,
         range: Range<usize>,
     },
}

// After: fields extracted to a separate struct
// and shared between enum variants
pub enum Payload {
     Module(Metadata),
     Component {
         metadata: Metadata,
         children: Vec<Payload>,
     },
} 
pub struct Metadata {
    name: Option<String>,
    producers: Option<Producers>,
    author: Option<Author>,
    description: Option<Description>,
    licenses: Option<Licenses>,
    source: Option<Source>,
    range: Range<usize>,
},