previously we used the format:
't' <utf-8 term> <byte separator> <16-bit field id> <utf-8 docID> <byte separator>
now we have moved the field before the term, resulting in:
't' <16-bit field id> <utf-8 term> <byte separator> <utf-8 docID> <byte separator>
this means now instead of all fields with the same term being grouped together
all terms within the same field are grouped together
this allows us to enumerate the terms used with a field
this allows us to implement prefix search, and possibly improve numeric range queries
removed analyzers (these are now built as needed through config)
removed html chacter filter (now built as needed through config)
added missing license header
changed constructor signature of filters that cannot return errors
filter constructors that can have errors, now have Must variant which panics
change cdl2 tokenizer into filter (should only see lower-case input)
new top level index api, closes#5
refactored index tests to not rely directly on analyzers
moved query objects to top-level
new top level search api, closes#12
top score collector allows skipping results
index mapping supports _all by default, closes#3 and closes#6
index mapping supports disabled sections, closes#7
new http sub package with reusable http.Handler's, closes#22