Add `pg_query::summary()` #155

Job	Run time
Check file formatting and style	15s
ci (stable, windows-latest)	2m 42s
ci (stable, ubuntu-latest)	1m 24s
ci (nightly, windows-latest)	3m 36s
ci (stable-x86_64-pc-windows-gnu, windows-latest)	2m 50s
ci (stable-i686-pc-windows-msvc, windows-latest)	2m 36s
ci (nightly, ubuntu-latest)	1m 26s
	14m 49s

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