Make schema_builder API fluent.
fix doc serialization and prevent compression problems

u64 , etc. shoudl return Resutl<Option> now that we support optional missing a column is really not an error
remove fastfield codecs
ditch the first_or_default trick. if it is still useful, improve its implementation.
rename FastFieldReaders::open to load


remove fast field reader

find a way to unify the two DateTime.
readd type check in the filter wrapper

add unit test on columnar list columns.

make sure sort works