Typed map<K, V> support is limited to HashMap with the default hasher #14832

Today map<K, V> can only be used through the typed API and bindgen! as HashMap<K, V> with the default hasher (std's RandomState, or hashbrown's default hasher without std). A few consequences:

• There's no support for BTreeMap, custom hashers, or getting at the raw entries. Vec<(K, V)> doesn't work since it only typechecks against list.
• bindgen! has no way to choose the type (no map_type option, and with doesn't apply to map aliases).
• additional_derives: [Hash] (or Ord) fails for any record containing a map.
• Lowering iterates a randomly-seeded HashMap, so the order of entries the guest sees changes from run to run, which is surprising for embedders following the deterministic-execution guide.
• Lifting deduplicates with last-wins semantics, which is where Duplicate keys in map<K, V> leak handles #14812 comes from.

For comparison, guest-side wit-bindgen has a map_type option plus a small WitMap trait, with BTreeMap as the no_std-friendly default and HashMap available behind std. Would something similar make sense here: impls for BTreeMap and HashMap<K, V, S>, an entries-preserving type for "just give me the list", and a bindgen! option to pick between them?