Add functionality to search on an iterable of references.

[Blakeinstein]

Searching collections other than slices, and a derive macro for Fuseable.

Added

• search_text_in_fuseable_refs searches any iterable of Fuseable references — a
  HashSet, the values of a HashMap, a BTreeMap, a Vec — with no intermediate
  copy of the collection. Results borrow the matched item, so there is no index to map
  back to an unordered collection. Closes #18.
• search_text_in_fuseable_refs_with_chunk_size_rayon and
  search_text_in_fuseable_refs_with_chunk_size, the rayon and async equivalents.
• FuseableRefSearchResult, the result type those return.
• A blanket impl Fuseable for &T, so &[&Book] works wherever &[Book] did.
• An optional derive feature, re-exporting #[derive(Fuseable)] from the new
  fuse-rust-derive crate. Mark searchable fields with #[fuse], or
  #[fuse(weight = 0.3)] to weigh them; unmarked fields are ignored.
• FuseProperty now derives Debug and PartialEq, like the other result types.

Fixed

• Chunked searches reported chunk-local indices: every chunk restarted at 0, so with
  chunk_size: 1 every result claimed index 0.
• Chunked searches scored differently from the sequential search. They omitted the
  0.001 floor applied to an exact match on a full-weight property, so the same input
  produced different scores depending on which entry point you called.

Changed

• The scoring core now lives in one place, shared by every Fuseable entry point, so
  the sequential, chunked and reference-returning paths cannot drift apart again.
• Bumped syn to 3, iced to 0.14 in the search-bar example, and refreshed the lockfile.

Breaking

• The blanket impl Fuseable for &T conflicts with a hand-written
  impl Fuseable for &LocalType where LocalType: Fuseable. Delete the hand-written
  impl if you have one.
• Indices and scores returned by search_text_in_fuse_list_with_chunk_size and
  search_text_in_fuse_list_with_chunk_size_rayon have changed. Both were bugs; the
  chunked results now match the sequential search exactly.