Improve meta fields

[koheiw]

If tokens_compound() is used, it should be recorded in the meta field, probably in ngram.

> attr(tokens_compound(tokens("aa bb cc"), phrase("aa bb")), "meta")$object
$unit
[1] "documents"

$what
[1] "word"

$ngram
[1] 1

$skip
[1] 0

$concatenator
[1] "_"

$summary
$summary$hash
character(0)

$summary$data
NULL

Also, we probably do not need summary any more.