feature: support multi-expression for same field in one conjunction

1. support multi expression for same field in one conjunction
2. flat-cursor and faster-skip-to got about 30% performance improve

Merge branch 'master' into stable_for_byted

support customized sep for ac query sentence

hotfix: concurrency access map error (#7)

1. fix: concurrency access Default map panic

feature: new collector api for reducing memory allocate (#6)

* feature: result document id collector

1. user/client can specify a collector to hold temp matched document id

* rename and simplify code

Hotfix/negative bug (#2)

* dig negative bug

* fix negative bug
name refactor fro design reason

a more effective way to retrieve doc. (#1)

* finish interface for compactedIndex and ksizedIndex

add more clear usage note and assert for invlid document

fix skip error

update README.md and module name to match the usage of MODULE mode