Show timing information

[krlmlr]

for the 6 slowest packages.

Example output (source):

Slowest checks

	package	check_time
3	AnnotationHubData	4837.7
4	AnnotationHub	3921.7
36	GenomicFeatures	3660
28	ensembldb	733.3
62	OrganismDbi	674.9
42	lumi	484.5

Motivation: A sudden change in timing may hint to a bug; ignore notoriously slow packages.

[hadley]

I think this would feel better as a separate function writing to (e.g.) timing.md.

[krlmlr]

Example: https://github.com/rstats-db/RSQLite/blob/8fbcff79424c73605fab99997ca8972ce16d5c6d/revdep/timing.md