feat(storage): support char column reader

[skyzh]

Signed-off-by: Alex Chi iskyzh@gmail.com

This PR adds fixed char block iterator and column iterator. Now we can:

create table char_test(v1 char(20) primary key not null, v2 char(20) not null)
insert into char_test values ('test', '233333')
select * from char_test

Close #115