Add iterators, incremental search, and collectable labels

[shanecelis]

This library remains a constant companion. I've reworked my previous pull request significantly. I'll include my write up from the CHANGELOG that explains the changes as well as I'm able.

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• Use iterators for search results.

  Benefits being that they're lazy, short-circuitable, and require less memory.

let a: Vec<Vec<u8>> = trie.predictive_search("ech").take(10).collect();

• Allow Label collection type to be specified.

  This includes machinery in crate::try_collect to allow us to collect into String directly.

let a: Vec<Vec<u8>> = trie.predictive_search("ech").collect();
let b: Vec<String> = trie.predictive_search("ech").collect();

• Add incremental search.

  Let's the user build their query one label at a time.

  let mut builder = TrieBuilder::new();
  builder.push("a", 0);
  builder.push("app", 1);
  builder.push("apple", 2);
  let trie = builder.build();
  let mut search = trie.inc_search();
  assert_eq!(None, search.query(&b'z'));
  assert_eq!(Answer::PrefixAndMatch, search.query(&b'a').unwrap());
  assert_eq!(Answer::Prefix, search.query(&b'p').unwrap());
  assert_eq!(Answer::PrefixAndMatch, search.query(&b'p').unwrap());
  assert_eq!(Answer::Prefix, search.query(&b'l').unwrap());
  assert_eq!(Answer::Match, search.query(&b'e').unwrap());

  If your search can be O(log n) instead of O(m log n), do that.

• Add Trie::postfix_search()

• Add map::Trie::exact_match_mut() to mutate map Values

• Add Trie::longest_prefix()

  Find the longest prefix. This is the kind of behavior one would want to implement tab completion for instance.

• No longer panics on zero-length string queries.

  Previously a zero-length query would instantiate the entirety of the trie essentially uncompressed. Now, however, an iterator only allocates one word at a time, and one can limit their search results to avoid whole trie collection.

let b: Vec<String> = trie.predictive_search("").take(100).collect();

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Of these changes perhaps the postfix_search is superfluous and could be omitted. (It's used internally but maybe shouldn't be exposed.)

The biggest change is that searches from crate::Trie do not return a Vec but an iterator and is certainly a breaking change. However, I think the benefits are worth it.

Happy to hear your thoughts on the changes. Thanks again for this very useful library.

Oh, and this also makes Trie clone-able provided the other two PRs to louds-rs and fids-rs are accepted.