Tantivy 0.12

• Removing static dispatch in tokenizers for simplicity. (#762)
• Added backward iteration for TermDictionary stream. (@halvorboe)
• Fixed a performance issue when searching for the posting lists of a missing term (@audunhalland)
• Added a configurable maximum number of docs (10M by default) for a segment to be considered for merge (@hntd187, landed by @halvorboe #713)
• Important Bugfix #777, causing tantivy to retain memory mapping. (diagnosed by @poljar)
• Added support for field boosting. (#547, @fulmicoton)